That’s an excellent question … with no easy answer.
On the “push hard” side is the fact that you probably need the money to keep the gym going until we emerge from the lock-down (whenever that will be), and to survive yourself. All of your customers will appreciate the online offering, and some will be in a position to pay, so you should encourage them to do so.
On the “softly softly” side is the fact that some of your customers will not be in a position to pay due to their own financial constraints. They will, however, appreciate your services being available AND free. Many companies with a service that can be offered online are choosing to make them available for free, counting on the goodwill generated to transition to paying customers when we recover.
Being too hard on the “Donate Now” message might make them feel bad about not being able to pay, and therefore reluctant to participate.
So the key is in your messaging. Point people clearly to the donate button, explain your rationale for requesting some payment, if possible, and stress that it is not required.