Co2 is stored as a liquid, air is obviously stored as a gas. You can't fill a Co2 with compressed air. You can, however, fill a compressed air tank with Co2, though it's hard to imagine why you'd want to (people used to do it in the shoebox shocker days, because the shockers got terrible efficiency on air, and an 88ci air tank holds a lot of Co2). The pressure that you're measuring on the gun won't change between Co2 and air (not much anyway), Co2 generally outputs at around 800 psi, and most preset air systems output at 800 or 850. A 1200 psi gauge will be more than sufficient for your purposes.