Gas Operated Air Guns are interesting. But are they practical? Let's review...
The fact that a slide can be manipulated by the same gases used to fire a 4.5mm BB in the GBB air pistols we see today is pretty cool, like, the realism and such. But would be apply it to home made air guns? My answer, probably not...
it's for aesthetics really, a hammer strikes a valve the same way a hammer would strike the firing pin or primer on a bullet casing and it sets of the ignition, the bullet flies out, there's fire and smoke and loud noises, but in a GBB gun, this all happens but with, yes obviously BB's. It's realistic and even has some kick, people love them, especially for airsoft wars.
How do they even work?
Well a hammer (cocked when the slide is racked back), flies forward and opens the air valve, high pressure gas moves up into the chamber where it appears to send the slide back and fire the BB in one equal motion resulting in blow-back, it's not all for show as the slide cocks the hammer again and obviously we get a high-speed shot from the gun.
But using gas to cycle a home made air gun?
There's no point, with the introduction of trigger-actuated valves and chambers, you can fire BB after BB or pellet after pellet by simply using your trigger to open and seal a valve doing all that good stuff. You save yourself a lot of trouble in terms of complexity.
Don't try to make your home made air gun gas-operated, it's impractical, it's cool, but it's not worth the trouble.