How to respond to a fake review for a Gym

First, decide whether it's genuinely fake, off-topic, from a competitor or someone who was never a customer, or spam. If so, flag it to Google (it can be removed for a policy violation) and, if needed, appeal with evidence. A truthful-but-harsh review isn't "fake" and won't be removed.

Removal takes time, so reply in the meantime, calmly and factually, for future readers. State that you have no record of the visit and have reported it, and invite a real customer to reach you. Never get emotional or accusatory in public.