The gagging is likely a result of swelling in the back of the throat from the radiation. Gagging is a reflex which triggers anytime there's stimulation at the back and sides of the throat - the movement of the swollen tissue when swallowing could result in the gagging. You're getting your treatment at Swedish, right? - please ask your onc. for a referral to Joanne Fenn Speech Language Pathologist at Swedish - she specializes in Head and Neck cancer. A dietian cannot diagnose or treat a swallowing problem only an SLP can.
Good luck!