First of all, you need to prove your innocence first. So, you can ask the gym trainer to check the CCTV recordings. Once he will see the recordings he will get to know that you are innocent. Then you can file a complaint against her for defaming you. You can only complain about her to the trainer. The court doesn’t usually take this type of case if it’s not so serious. Although, the court does take the case against slander.