I am a certified group trainer and a PT and I have worked in gyms as well as offered private sessions. The gyms don't pay much but offer a safety net -- freedom from liability, equipment, a steady stream of clients and some job security to start.
I did not earn enough just working at a gym even though I was full-time. I had to hustle and freelance and made the most pay when I taught group classes or had repeat private clients. To gain clients I put on free classes, networked with other instructors, taught for fund-raisers, formed fitness groups, etc. Oh, and my gym allowed me to train non-gym members at their facility (of course for a guest fee). Many gyms wont allow that so check first.
Lastly, be prepared for the inevitable drop-off of your clients. Only the people who are completely into fitness stick with a programme. And they are the ones who eventually will not need your services!
Don't get me wrong -- I love it and it keeps in motivated and fit. It just doesn't pay many of the bills...