You can regulate compressed air down to pretty much any temperature you want. You just can't cram much of it into a magazine. Airsoft propellants are mostly stored in a liquid state which makes for a very high compression factor. Compressed air is normally used as an ideal gas which means no liquid phase. If you regulate it down from your high pressure storage tank, you only transfer gas which makes for a very poor compression factor if you regulate to a sane pressure which wouldn't blow open a GBB mag.
Unless you use a remote connection to a larger tank, you usually can't get much more than 3-4 shots into a GBB mag and each shot brings rapidly degrading performance (pressure drops rapidly with each shot).
Want nearly free GBB gas?