The issue with offering any unsolicited, or even solicited advice, is that the liability for any injury that may be sustained then moves away from the gym to you. You may well offer the correct way to use the equipment, but if an injury has been sustained anywhere along the line beforehand, and the user sues, the gym or its insurer could quite reasonably come after you.
By the way, I am not talking about simple injuries, but incapacitating ones, such as spinal issues, tendon and ligament ruptures, etc.