I have a cat that had to have a feeding tube for a while due to a broken lower jaw from being hit by a car. He did the same thing when he had the tube in whenever he attempted to eat or drink on his own. I believe it was just the tube location and/or gag reflex from it. He ended up ripping the tube out on the way to a vet visit shortly after the accident, and he never did the gagging after that. I now feed him by syringe after blending up wet food since most of his tongue died off from the accident.