By gassing you, do you mean that when you turn the pumps on, the water would get very cloudy/airy looking and emit a very strong Clorox smell?
If that is your problem, what is being used in the spa, chorine, bromine, other?
I had a problem like the one I described and was informed that it was due to a buildup of bromine, where I left the floating bromine capsule in the water and caused the problem. I had missed about a week of "soaks" and the bromine built up in the water. I was also told that bromine doesn't "gas" out like chlorine does. It just keeps getting strong, and will get to a point where it can be a real health risk.