In a situation where by you taking or using someone else's property or service they are not losing from your usage, such as when you borrow someone else's gym-membership card without permission, can you take advantage לכתחילה? Would it be more lenient by non-Jews?
Tosfos Bava Basra 12b says it is obvious that it is only bedieved.
Besides that, it is very clear in the Talmud that this does not apply to a commercial operation (such as a gym), where what is an irrelevant loss in a private setting is relevant, as the use wears out the property. So one cannot just sneak into a hotel and sleep there, even though the room is vacant anyway. So in this case, even bedieved it is a problem.
Now if you have paid for a gym membership, you just wanted to use the friends card because yours wasn't available, that might be a closer question.