im not sure about this. personally, i try to gym 1 day before or on the day of my flight, and i expect people who care about their fitness to do something similar, so gyming during a layover is low on my list. furthermore, im not keen to gym during a layover because of time constraints, risk of missing flight, hassle (bring extra workout clothes & shoes and bathe after), etc. i also tired and dehydrated from my earlier flight. instead, i prefer to walk around to get my blood pumping and try some local food.