Gargoyle (Tyranids)
"Fly my pretties. Fly!"
- – The Wicked Witch of the West
A gargoyle (Gauntii avius) is a winged variant of the termagant and possesses the same armament and same chitinous shell, except for a venom gland that allows it to spit acid. When not under the command of a synapse node Gargoyles are known for being weak and cowardly, shying away from combat. However, they are also described in the fluff as being lethal in a confined space, with claws, spikes, barbed tails, flapping wings, and venom flying everywhere. They typically like to dive upon exposed and weakened units like Imperial Guard command squads, as it can be rather difficult to fight off a swarm of acid spitting, gun toting bats that view you as a tasty slice of cake. Other preferred targets include isolated characters, back-field support units such as heavy weapon teams, and below-strength squads. Any target that is alone, vulnerable, and/or weakened would be ideal preferred prey for a swarm of Gargoyles.
Basically, imagine the Bio Raptors from Pitch Black. But they can see. And light doesn't hurt them. And they have guns. And spit blinding venom like the Dilo from Jurassic Park. Yeah, pretty fething scary.
On the Tabletop
Prior to 8th edition, Gargoyles were Termagants with wings and a hefty points increase. They were rather unpopular, but not any more so than the rest of the Tyranid 5th-7th Edition Fast Attack choices. Now in 8th/9th they are only 2 points more than a Termagant despite having considerably more capability. Even so, you still need to know what you're doing to get the most mileage out of these guys.
In game their main use is to Deep Strike right behind enemy lines, away from fighting, and then attack things like command units, snipers, heavy weapon teams, and gun emplacements. They will wipe out these things through massed fleshborer fire, and if the unit is sufficiently depleted, dive in to melee and use their acid to finish them off.
An example of this tactic would be to use two winged hive tyrants with twin-linked brainleech devourers, supported by 30 gargoyles each, to rape the living shit out of enemy lines. Place them where the enemy has a crapload of important but vulnerable support units and then pelt them to death with so many dice rolls that they drown in them.
Really though, gargoyles are simply bats with guns, and use numbers (like all Tyranids) mixed with speed to force an opponent's death from aforementioned dice drowning. Thankfully, the newest Gargoyle models are now plastic, not the hellish metal ones, and as of pre-Codex 9th Edition they remain reasonably priced at 7 points per model. Gargoyles would be even better if they could use devourers, but then, flying dakka bats with acid spit would be absurd.