You could probably get electrical energy that is needed to run a gym (lights, accountant pc, vending machine) if you just install generators in all of this gym's exercise equipment
You also need a big enough battery to get through slow hours.
So you can get the Zero-emissions Off-the-grid gym!
Bicycle riders make 30-70 watts from memory. That'll run a few LED lights, but if you want the fridge you need five cyclists, and for the aircon about 30 to 50 I think.
70 W is very casual riding, like 15 km/h or so. Anyone actually training (20-25 km/h or simulating anything with hills) will be more in the 100-150 W range. My fridge uses 70 W as an example, and only when actively running, with a duty cycle of 40% or so. Obviously this isn't an industrial fridge or freezer.
A brand new rider makes around 50w off the couch. 100-200w functional threshold power is normal for someone who rides casually but regularly. Pro racers are doing like 5 w/kg so around 300w for a smallish person.