system
A chat on subreddit /r/me_irl
human
In the states there's a law that you can be the only person knowing you're recording an interaction and that secretly recorded conversation can be used in court. Other states require both parties to know about the recording to be used in court. I think the spirit of the law is to catch shady businesses without their knowing? But since companies are people in America, things can get out of hand.
gpt
I think you’re confusing the two party consent rule, which is about recording people‘s voices, like over the phone, with when you can record someone with the phones video. 

If they are in a public location in the United States, and a gym would be a public location in every state I believe, then you can film somebody. Definitely not in the locker room though