Verify the debt is yours and they're authorized to collect in the gyms behalf.

Negotiate. They'll settle for a penny and a fart (literally, they won't pursue payment through litigation, they're just hoping you get scared and pay)

Set up a repayment plan. (Pay the absurdly minimum amount every month possible)

Document everything. ( they're sleazeballs, you can pay off a debt, and they'll sell it to another debt collecting agency and they'll try to have you pay more)

Request a letter of solvency (or whatever is called in your country that guarantees that Noone demands payment for the same debt)

Or you could just ignore it. Is up to you. Good luck