the gas blowback is referring to the operation of the action, the gas propels the projectile and moves the slide or action to the rear to load another pellet. this type of action makes gives your gun the same operation as a real firearm. the gas gives your shots more power as well over the traditional spring powered airsoft guns.
its what you think, when you shoot a gas powered air-soft gun with blowback, gun will force you back a bit, like a real qun would. This may mean the gun is pretty powerful.
do i need the gas or does it automatically just blowback?
it needs the gas, it would be a powerful gun.