Thanks Sandert17. Sports centre is not what a gym is, and as regards a gym not being a sport, neither is a swimming pool or a golf course; you cannot place a sport on a map, only a place where sport takes place, so there should be an icon for a gym just as there are icons for swimming pools, hunting hides(?) and golf courses.

Thanks for the replies. There must be millions of gyms on this ridiculous planet--I can't believe I'm the first person who wants to add one to openstreetmap, but it seems that that is the case. The wiki does not have an icon for it, and I am not talented enough to design one, so I suppose I'll just let it be.