Gas gun

A gas gun can be: A projectile-firing gun powered by compressed air:
Air gun
Airsoft gun
A Less-lethal launcher, especially when loaded with CS gas grenades.
A pneumatic cannon for projectile-firing test equipment such as a birdstrike simulator
A gun that fires a stream of gas. This can be for these purposes among others:-
The generate a loud noise as a bird scarer
For propulsion in space: see Manned Maneuvering Unit#NASA "gas gun"
Light gas gun, a velocity generating device used in physics experiments.