Gas airsoft replicas are powered through their magazine which are filled with gas. There are two types of gas powered replica's. The GBB (gasblowback) and the GNB (gasnon-blowback) you can distinguish those two by the pressure it gives during each shot. The GBB gives a backward pressure after a shot where the GNB doesn't give any pressure at all. Keep in mind that you need to fill up the airsoft magazine regularly with gas.