Well, the actual air-soft gun or green gas gun is an entirely separate and unique training aid/gun. It's replicates the actual firearm, so you should still be able to use your existing holster, but it's a green gas gun unto itself.
Other simulators are available that either replace an actual gun's barrel, or insert into the actual gun's barrel (LMTS for example) but those are different training simulators all together than the airsoft/green gas training guns, and generally more expensive too. Those simulators (LMTS) use a combination of infrared light and receptors (some computerized, some stand-alones) and record "hits" accordingly. However, because they only "click" through the the gun's function, there is no blow-back or recoil or simulated "firing" beyond the "click" of the trigger mechanism which actuates the 'firing' of the infrared beam, which results in immediate feed-back on the infrared receptor. Generally speaking.
If you've got a training budget like the Army's, LMTS DOES provided specialized "blank ammo" and specialized uppers for M16/M4, and even ammo and "blank barrels" for M249's, so then you can use LMTS for training AND get the full functionality of the weapon system - which is outstanding training. I used to run LMTS courses for light machinegun and "fire" inside a warehouse. No environmental considerations, moderate hearing protection was used, and the training was great. A similar system was available for the M9, but again - we're talking Army training budget. I don't know how many "regular Joe's" could afford the Blazer uppers for the M-16, or the blank barrel for an M249, let alone the specialized blank ammo.
It's a far cry from the old crappy blank adapters and blank ammo that may or may not function through a weapon system on any given day.