Gauss
FIRST AND FOREMOST: It is pronounced such that it rhymes with "house".(unless your a classy americunt and just don't give a Shit)
Gauss is the last name of Carl Friedrick Gauss, one of the greatest mathematicians in history. His work in calculus was (and is) invaluable to the study of electromagnetism, so he got a unit of magnetic field strength named after him. It is universally agreed that "Gauss" is a cool name, and so you can bet dollars to donuts that any sci-fi gizmo with any sort of vaguely electromagnetic theme will have the word "gauss" in it.
By the way, if an electromagnetic weapon doesn't have "gauss" in its name, it's a fair bet that it has the word "Tesla" in it, after another cool-sounding unit of magnetic field strength, itself named for Nikola Tesla, a famed inventor and scientist who did cool stuff with electromagnetism.
"Realistic" gauss weapons come in two flavours: rail-gun and coil-guns. Rail-guns work by having a really long pair (or pairs) of conductors, and passing opposing currents through them and the ammunition. It works like an electric motor that is unwound into a straight line. Cheap to manufacture, awesome velocities, but it is energy-hungry, needs to be really long and causes a lot of stress on the materials. It's not to be confused with the Peasant Railgun. Coil-guns work by surrounding the barrel with a sequence of coil magnets that are switched on to pull (and then reverse current to push) the ammunition. Since the ammunition doesn't physically touch the mechanisms, the gun lasts longer and there is better control over the speed, but the control mechanisms are very complicated and the overall weapon is weaker than a railgun of the same current draw. Of course, once you get into soft sci-fi, all bets are off; expect to see guns that shoot lightning.
When used as a name, "Gauss" is capitalized. When used as a unit or adjective, "gauss" is usually lowercase (though the abbreviation for the CGS unit is a capital "G").
Real-Life
The US Navy has successfully tested 32-megajoule railguns. The friction from the ammunition moving at these velocities turns the air inside the barrel into plasma instantly. They were planning to build a 64-megajoule testfire, but they lost funding. This might have to do with how they had to build an entirely new railgun after every third shot; the rails wear out quickly due to the heat the projectile produces, and no known material is heat-resistant enough to withstand being worn out in this manner and also capable of conducting electricityto allow the mechanism to work in the first place. You also need a really strong generator to keep it powered as well, or the projectile will be propelled with less force.
If you see a carpenter using an "electric nailgun," it's actually a coilgun they're using.
Rifts
In Rifts, a "Gauss cannon" is just a synonym for "machinegun that does MDC damage." The artwork for railguns always shows them as ejecting cartridges and being round, with no rails in sight ... it's not like anything else in Rifts even tries to be rational, so just roll with it.
Traveller
In Traveller, Gauss weapons are slugthrowers that use 3-5.5mm (.11 to .21 calibre) slugs or flechette needles. They're the preferred rifle and sidearm for TL12 armies (Tech Level 10 in GURPS Traveller); at this tech level, the preferred heavy weapons will be laser or plasma. Until these non-chemical sidearms are in use, ship boarding parties in space prefer using sabres and melee for combat. The exception is zero-G environments, where recoil on any slugthrowers (gauss or not) are as much a hazard for the wielder as the target.
Warhammer 40,000
Being set in the grimdark future, several Warhammer 40,000 races use Gauss weapons.
Tau
The Tau use railguns as the primary heavy weapons on tanks and walkers. In these cases they are very powerful and penetrating (as in real life) as the acceleration is limited only by the amount of power that can be input and how much the weapon can handle without exploding or melting.
- Railgun: The Railgun is the iconic Tau heavy weapon, mounted on Hammerhead Gunships (and Battlesuits, in previous editions) to cause massive damage to enemy armor (72", S10, AP1). A Broadside Battlesuit Team used to be able to carry up to 3 twin-linked railguns in one Heavy Support slot (nerfed to the S8 heavy rail rifle, see below, in their 6E codex), while the Hammerhead can only carry one, but the Hammerhead has enough ammunition capacity to also carry a S6 AP4 Large Blast submunition round for dealing with infantry blobs. (Squad of guardsmen a turn anyone?)
- Heavy Railgun: Because the regular railgun wasn't enough for putting down the really big targets (like Titans), the Earth Caste developed the Heavy Railgun (110", SD, AP1) for their super-heavy vehicles. It was originally designed to blow up space ships, as Tau never expected anyone to be stupid or crazy enough to build a big enough land-based vehicle to justify this weapon's use against it. Then they encountered Ork gargants and Imperium titans, and figured out that most inhabitants of the Galaxy aren't sane or reasonable. The Manta mounts two of these, while the Tiger Shark AX-1-0 mounts a twin-linked pair. Like the Hammerhead railgun, the heavy railgun can fire a pie-plate for destroying massed infantry formations, but the heavy railgun's submunitions are S7 AP3 and the blast is 10" across.
- Rail Rifle: Emboldened by the success of vehicle-mounted railguns, the Earth Caste built a Tau-portable railgun weapon, called the "rail rifle." It debuted in Warhammer 40,000: Fire Warrior as a prototype weapon; it soon got rules in Chapter Approved, and by the next Tau Empire Codex, it was a regular part of the Fire Caste's armory, used by Pathfinders and drones as a sniper weapon. Incidentally, while it was in Chapter Approved, the Rail Rifle was subject to "Gets Hot!" like a Plasma Gun, but the Earth Caste improved the safety margins on it such that blowing up in the user's face is no longer a concern by the time of the 4th Edition codex. 6th edition further uplifted the Rail rifle, turning it from Heavy to Rapid Fire and making it AP1, meaning it now could oneshot terminators and light vehicles, be fired on the move and even could be fired in full-auto. It is mentioned in the fluff that once Rail Rifles would be advanced enough to mass-produce and do not eat all the ammo and battery pack in a matter of seconds (and seeing their progress rate from overheating backpack-powered heavy weapon to magazine-fed rapid fire rifles in a matter of a few decades it's not a long time to wait for), Tau command have a plans to use them as a standard issue Fire Warrior guns instead of pulse rifles. Thankfully, seeing as it would pretty much break the game with Tau basic infantry being able to steamroll everything in their field of view even more than 3E Necrons could, GW's refusal to advance the plot will not allow this to happen, by freezing their timeline on 999M41, before the rail rifle technology can be fully refined.
- Heavy Rail Rifle: A recent development wielded by next-generation Broadside battlesuits, it is less powerful than the Tau's primary railguns, though only moderately so (S8 versus S10). However, its reduced bulk allows it to be mounted in such a way as to more easily track fast moving targets, and thus it fulfills an anti-air role, where its slightly reduced strength is not a big liability as it is still plenty powerful enough to punch through most fliers with ease, and its double-barreled (twin-linked) setup increases its odds of hitting a target rather than its odds of wounding or total wounds deal after wounding. That said, it's still a considerable threat even to AV13 vehicles, since with AP1 the first penetrating hit would likely be the last.
- Heavy Rail Cannon Array: The latest advancement in bringing hyper magnetized death to the enemies of the Greater Good. The Heavy Rail Cannon Array is mounted on the KX-139 Ta'Unar Supremacy Armour, and is designed to take down Super-Heavy vehicles and Gargantuan creatures, alongside any infantry which may be attempting to close in on the suit. The Heavy Rail Cannon itself possesses a longer range than the equally destructive Heavy Railgun topping out at 120", S:D AP1, Ordinance (1), Blast (3") however it has been designed to aim at the heaviest points of an enemy and strike those points with enough force that should it wound the enemies own mass is turned against itself (roll twice on the Destroyer Weapon Attack table and pick the higher result when firing against Super Heavies or Gargantuan Creatures), and is paired with the Cluster Shell system which is used to launch sub-munition shells at nearby enemies (36", S:6 AP4, Apoc Barrage (4), pinning), in such a way that both weapons may be fired against different targets at the same time.
Necrons
Most Necron weapons have the word "gauss" in them, but they all shoot some kind of molecule-stripping super-lightning rather than a magnetically-accelerated projectile. The Third Edition Codex stated that it was a magnetic field that 'pulled' the target towards the gun at the atomic level, one layer at a time in an effect like flaying, despite the fact that magnetic fields do not and can not do this (gravity can, so the most plausible explanation is that these guns shoot gravity; the descriptions are consistently similar to how a black hole's event horizon works). In the Third Edition Codex, gauss weapons automatically wounded infantry and automatically caused glancing hits to vehicles on a to-hit roll of 6. This was devastatingly powerful at the time, as it meant that Necrons could chew through vehicles like no tomorrow, but was rather reduced in effectiveness in Fifth Edition, as most vehicles could no longer be destroyed through glancing hits alone, though with the introduction of Hull Points in Sixth Edition, gauss weapons are once again a lot more dangerous to vehicles (three glancing hits -- which a full squad of 20 Necron Warriors can easily supply -- is enough to wreck most regular vehicles and expose a super-heavy ). That said, Gauss weapons were reduced in effectiveness by the Fifth Edition Codex, which removed the auto-wound property (anti-infantry effectiveness was moved to their new new "Tesla" weapons). However with the advent of the 7th edition codex gauss weapons have regained their auto-wounding on 6's, which wasn't much of a buff since those weapons already did wound on a 6 in most cases, leaving that rule only being useful against Gargantuan Creatures.
- Gauss flayer: The trusty gauss flayer is the standard weapon (in fact, the only weapon) wielded by Necron Warriors. Its stats are equivalent to the bolter, with the additional "Gauss" rule mentioned above. Ghost Arks and Doomsday Arks mount an array of five of these guns on each side.
- Gauss blaster: The gauss blaster is the standard weapon of Necron Immortals; it is basically a Gauss flayer with slightly improved strength and armor-piercing capability. A unit of Tomb Blades can also choose to take twin-linked pairs of them to specialize in vehicle-hunting.
- Gauss cannon: The iconic weapon of Necron Destroyers is their shoulder-mounted gauss cannon. Though it's only as strong and long-ranged as a gauss blaster, it has more shots and will tear through all but the toughest armor suits.
- Heavy gauss cannon: Necron Heavy Destroyers get heavy gauss cannons, and Triarch Stalkers can choose a twin-linked set of them as a primary weapon. They only get one shot, but it's as powerful as a lascannon.
- Gauss flux arc: The gauss flux arc is basically like the gauss flayer, but firing more shots at a time. Monoliths mount one at each corner, Ghost Arks mount one at each side, and they are capable of choosing their targets independently.
Eldar
Eldar in general do not make wide use of magnetic weaponry. Their Shuriken and Shard weaponry function similarly to magnetic weapons, but rather than using an electro-motive force they use miniaturized grav-generators similar to what keeps skimmer vehicles airborne (at least according to the 2nd Edition fluff on shuriken weapons which we have no reason to believe has changed.) Essentially, they make gravity inside the barrel point to the end of the barrel as "down" as though they were on a particularly heavy gravity well, and the rounds "fall" out through the end of the barrel with an intense acceleration, keeping their momentum once they align back with the normal gravity outside the gun.
Imperium
Strangely enough, heavy bolters use coilgun tech[Citation Needed] to further accelerate its bolts, so they can use much lighter ammo with less gunpowder (as real life gyrojets have shown, more "kick" is needed to make it viable at ranges below a dozen or more meters). Also, the NOVA CANNON is sometimes described as fuckhuege railgun, though this is only one of a half dozen completely different explanations of how the NOVA CANNON actually works. (which is actually surprisingly fluffy, when you think about it.) Macro Cannons, the Imperium's go-to for ship-to-ship broadsides each utilize a massive coil gun in the outer muzzle shroud to accelerate their shells even further.
StarCraft
In StarCraft, the main weapon of the Terran marines is a gauss weapon referred as the Gauss Impaler Rifle, and their design varies between the first and second games, where in the first they looked like pump action shotguns (big ones almost as tall as the user) while in SCII, they have a much more boxy shape. Despite being magnetically fired, the games still depict them as having muzzle flashes and for some reason using .50 caliber cartridge ammunition, despite the fact that gauss guns can do away with the cartridges by just directly launching the slugs, but Starcraft is hardly the only thing to ever inaccurately portray a realistic gauss weapon. (And they could probably handwave the muzzle flash as a split-second of plasmized air, since the slug is traveling so fast).
Despite being one of the weakest military grade weapons, and that's an understatement given that Impaler rifles are almost at par with the laser and photon weapons commonly mounted on Terran and Protoss gunships, Impaler rifles still can pierce through about anything, and though the little holes a single Impaler leave in enemy armor do not really bother giant humanoid battle robots, towering War of The Worlds-esque death automatons, and bone/chitin plated monsters the size of small house, massed gauss fire could and would bring down pretty much anything just by turning it into oversized swiss cheese until it collapses under it's own weight. That's why basically every Terran armed force in the galaxy worth a damn usually send marines in hundreds to drown their enemies in bodies and gauss fire. How they're able to sustain those losses on a constant basis is anyone's guess They just use brainwashed convicts drugged with steroids and adrenaline (which eventually kills them, if they somehow survive on battlefield, which they usually don't), and considering low life level anywhere outside core worlds (and even on some of core worlds), they have almost unending supply of criminals.
Although, how an infantry rifle could bring down a Terran Battlecruiser or a Protoss Carrier, which are essentially massive ships designed for large-scale ship-to-ship combat, through massed fire tends to make you wonder: What the hell?(crunch does not need to reflect fluff)
Battletech
In the Battletech universe, Gauss Rifles are some of the most powerful solid-based weapons. Packing the power native to Heavy Autocannons, which generally suck at range, and the range negligible heat generation comparable to light autocannons, which are long-ranged and produces little heat but lack the sufficient punch to threaten heavily armored 'mechs. They're capable of smashing a MadCat's cockpit off from long range with a single, well-placed shot.
The Gauss Rifle however, suffers form a few problems: One is that they cycle rounds much slower than conventional autocannons, limited ammunition count, and that they're heavier than most weapons. Also, while their ammunition is inert and won't explode from heat or critical hits, the weapons themselves are a bit unstable, and will explode if they suffer critical hits (though for much less damage than most ammo explosions).