system
A chat on subreddit /r/me_irl
human
This isn’t true.

Each individual gym is allowed to make their own  policy regarding filming.

You arent going to be arrested for this unless you are filming in the locker room and post someone’s nudes. You arent going to be sued unless the other person can prove you caused them significant harm
gpt
All standard gym contracts outline no filming or photography. Breach of privacy is not the same as slander or libel. Just like breach of contract. You sue the gym, the gym sues the member in question.