Usually, most gyms have CCTVs in their rooms, so those can be a big help in proving your innocence. As you said a female gym trainer is falsely accusing you of molesting her then evidence plays an important role and you need to collect as much as possible which can prove your innocence. Ask other staff to check the CCTV recordings, so that everybody can see the truth and you can collect evidence. But before all that, you need to ask the gym trainer first, why she is falsely accusing you of molestation. Try to find out the truth behind her lie.