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.