My Husband also has an extremely quick gag reflex.
The best thing for him was sedation just before radiation, it calmed us both down!
All during his rad treatment I would stand beside the Tech watching the screen, ready to jump to his rescue , except when one day we had a new guy in and I had to tear him a new one when he tried to stop me, silly boy! Just knowing I was doing that helped him relax.
best of luck to both of you
Marica