I think it’d be really cool to have something like edublogs.org or cobot.me - but Discourse-based - for gyms. In a similar vein as what @mcwumbly already pointed out though, surely there’s gotta be some gyms out there that already do something like this at least to a small extent? You should track down as many localised & gym-related discussions you can possibly find and study them.
Btw, the reason @sam brought up email is because it’s a great onboarding tool. Many users might have reservations about hopping onto yet another platform, but if your help@mygym.com address creates PMs of incoming messages and stages accounts for customers’ emails, you get your staff comfortable with the platform while gently introducing the idea to customers that “it’s cool that you care enough to interact with us; maybe you’d like to interact with other gym members as well?”.