Which "reflex sight" are you referring to, specifically? They all mount differently.
Doctersight, Tasco/Optima type mini-RDS sights use a mount that slides in to the rear sight mount. As far as I know, the airsoft Glock has a very close fit to real steel sight mounts. Typically, the mounts for these sights right around $49.95 USD and if my hunch is correct, should slide on to an airsoft slide with little to no modifications.
The RDS is then screwed down on to the mount.
Similar idea as what I have on my Hi-Capa (except, because the rear mount is airsoft-specific, I used an airsoft mount that pretty much costed as much as a real steel mount):
