I've only been to the GF, but they are owned by the same company. The way it worked for me, is that when I arrived, I left my room key at the front desk. I was free to you use the lounge and facilities for as long as I wanted. When I was finished with my treatment and ready to check out, the desk had my bill printed out and waiting for me. I chose to pay with my room key and there was a spot to fill in the tip amount, or they provided a small envelope that I could insert cash into. I use the same standards as restaurants, 15% or more based on my experience. I thought that I had read recently where someone said the gratuity had already been added to her bill. Just double check that.
Tammi