After 125 dives I find I have develped the same trouble when I am in the water for a good length of time. I tried different mouth pieces, taking the regulator out and getting a little water in my mouth (this wouldn't work to well in salt water). I bought a product called Oasis; it's a moisturizing mouth spray for dry mouth. I has stopped the gaging for me. I leave it on the pool deck in case I need it.
Becky
Becky