My girlfriend and I have used the gym at Marriott hotels in Amsterdam and Cambridge, England. We were often the only people there. To be honest, I don’t consider the risk of using the gym to be even worth considering. Clearly the risk of transmission is low if only two people are allowed in, and they clean between users. We are also in our 20s and I have covid antibodies.

Edit: if using the gym represents a risk to you on a personal level (immunosuppressive therapies, significant comorbidities etc) or you live with vulnerable people, then you shouldn’t be travelling or using a gym.

Second edit (my bad): using the gyms required booking a slot.