I'm not talking about Gym class in school I'm talking about a neighborhood gym that I'm a member at. I'm entirely serious about this and not trying to trump up a bullshit extracurricular, I've spent a good 1.5-2 hrs at the gym after school on weekdays since freshman year (going to be senior) and it is a pretty big part of my life, I've met a lot of my current friends there etc. However I am in no way a bodybuilder or anything that goes to competitions and as far as I know there is no documentation of my gym ( at that I would have access to) use so would it be a mistake to put it down as a major extracurricular on a college application?

I wouldn't put down specifically that you go to a gym every day as that sounds kinda stupid on a college application. I suppose if you're really hurting for extracurriculars you could put weightlifting down or even more generally just something like physical fitness, although I personally wouldn't but it probably wouldn't be bad either. And colleges don't really check to make sure your not lying about your extracurriculars, especially if they're not school sponsored ones, so don't worry about needing to be able to prove it or anything.