Sounds like you lucked out! Your, 'new' (used) G-21SF should work and shoot just fine - Nice going!
PS: Remove the slide and look to see if there's a slight silver-colored rub mark on the area over the top of the trigger bar. If you see one PM me; and I'll tell you what to do about it. Not to worry, though, your G-21SF is, still, going to be OK.
The G-21SF pistols, all, have a good reputation; there were very few reported problems with any of them - Like I said, I think you lucked out!