actually vapourised co2 (probably). sounds like you have a p68at or a pmi1 or the like. The tube for the paintballs doesn't have a spring, because it is "Stock Class" and requires you ro rock the gun back while cocking (pumping) it, in order to chamber a ball.
As for the gas, you need 12 gram co2 cartridges, like are used in pelet guns and some BB guns. there is a plug in the end of the tube under the barrel that holds the 12 gram cartridges)