A gast gun is a type of firearm that uses high-pressure gas to operate. Think of it like blowing up a balloon really fast and then letting go. The pressure from the balloon makes it fly around the room. The gast gun works in a similar way.
When you pull the trigger, gas is released from a canister and pushed down a tube. The gas pushes a piston that compresses a spring. When the piston reaches the end of the tube, it releases the spring, which moves forward and hits a valve. This opens a passage for the gas to escape, which pushes a projectile (like a bullet) down the barrel.
The process repeats itself when you pull the trigger again, and the gun can fire multiple rounds in this way. Gast guns are often used in paintball and airsoft games because they don't use gunpowder or bullets, making them safer to use.