Nearly impossible to free lance in a commercial gym. They don't allow it. If you are a trainer for a commercial gym the lion's share of the client's money goes to the gym and not to you. Also it helps if you look the part. Unless you are training seniors nobody wants a chubby trainer.
I was thinking of free lancing personal training and the owner of my commercial gym said no problem but he wanted a 50% cut. I have an amazing home gym but home owners insurance will not cover it. Personal trainer insurance won't cover a massive attorney led law suit against my home.
I only heard of one personal trainer success story. Met a woman in a library that said she has really high end clients in Manhattan. I don't know how she free lances but she must make enough to pay the gyms off. She lives in a really well to do area. Said she her business exploded from nothing and word of mouth.