My husband, who is a very similar "case" as you, same age, and course as you with the exception of that he had a total gastrectomy. He began experiencing this same thing right after he started radiation, and it continued for months until his doctor decided to take a look and did an EGD and found an anastomotic stricture, where the esophagus and duodenum are joined together, it was caused by scar tissue and radiation and he was able to dilate it and he has had some improvement. However, he still does have some episodes where his gag reflex will kick in when he is trying to eat. We were told that there could be some nerve damage from radiation that may or may not get better. Only time will tell.
I'm sorry you are going through this, we understand the frustration. I hope you find a solution soon.
God Bless