What do we call people who go to the gym?
Solution 1:
A term that I have heard, that seems to be widely accepted, and that I would use myself, is simply "gym goer". I don't think you'll find anything better than that, if you want a generic and neutral term. Google has 704,000 hits for it.
(I'm not a native English speaker, but I have been to the gym twice this week and plan on a third time, and I frequent web sites about strength training.)
Solution 2:
I'm afraid I can only provide two words: gym rat.