If it's a buffer type then it should be easy. Take the stock off (send us a picture and we'll tell you how) then use a screwdriver to unscrew the tube: It'll just come right off and you throw on the new sling mount. Again, if you could just send a quick picture then that would help immensely if you've changed the stock at all. You won't need to mess around with your gearbox or anything else so don't worry.
It depends on where you're looking to mount this as to what's compatible, but I assume you're talking about what we were earlier in the thread - buffer tube/recoil spring housing sling mounts. The buffer tube (the tube that comes out of the main body (receiver) of your gun (and which the stock sits on) is fairly generic if it's G&G so getting something that fits won't be hard.