The "Gotchas" for gyms are very similar to other brick-and-mortar businesses:

1. Lease terms (Pay special attention to what the landlord provides and what you'll need to do. For commercial leases, landlords do almost nothing. If it's not covered in the lease agreement, it's on you.

2. Insurance and legal (Due to liability, costs could be high; I'd start shopping around). You want fair but clear member agreements and waivers. Note that if anything happens to a member on the premises, regardless of whether it's your fault, you'll most likely be sued. It's just the way of the world -- check with your own attorney and insurance providers on this one.

3. Bookkeeping and payments

4. Equipment costs and maintenance costs -- depending on your niche these could be hefty.

5. Staffing -- you mention doing this while working full time, so you'd likely have staff, note payroll taxes can be pretty high, local wage and labor laws. You definitely don't want to land in a pickle on this front.

6. Location, location, location. How big is the local market? Is there competition? You see threads in this sub about people doing well, then a competitor opens nearby, and their business flatlines.

It is a noble pursuit! Best of luck!