A few times I left the airport and went to the nearest Anytime Fitness to work out!

Your target market isn't only layovers. Many times, I wish to go earlier to the airport to avoid traffic - a gym gives you something great to do before the flight in such a circumstance. Also, after landing, if you arrive too early for your hotel room.

Travel Frequency: ~2x a month
Airport Activities: Normally I eat a meal, usually at the Centurion Lounge. I definitely would enjoy a more active option.
Fitness Preferences: Free weights and elliptical are my usual go tos at gyms.
Gym Benefits: If there was a gym right at the airport, what benefits would you look for? Feeling better, sleeping well, and keeping fit in spite of a travel-ful lifestyle
Usage Likelihood/Concerns: I'd love to - but realistically it'd probably depend on how much I'd get out of my way from my gate. For instance - a landside gym at a far away terminal would be hard to justify.

Showers are definitely a must. A steam room would be WOW, and perhaps entry could be sold just for those two stops. I'd imagine you can make a killing selling a healthy to-go meal for the flight afterwards. You'd need luggage lockers.