Ben; Im into my 6th of 7 weeks rad / chemo. My taste was gone within 10 days and everything tasted of salt . Even water was like sea water. I know what you mean by gag. What it turns ut to be is the accumulation of the thick secretions in the back of the throat that acts just like you had put a finger into the back of the throat.... you know. anyway its more or less a mechanical thing not so much a true nausea but it sure isn't fun. I manage by swishing water in the front of the mouth and keeping the mouth moist then kind of hoof it from the back of the throat.
I geo a PEG about 2 weeks into therapy and have been glad I did. Couldn't take anything by mouth only sips of fluid. I did find that drinking some kind of soda water with flavor only ( no sugar ) helped break up secretions and also was ok to sip down. The PEG is your friend take good care of it and keep it flushed clear. I CAN taste a little coffee if I sip it... give that a try.
Everyone on this site will do their best to support you.... Need to talk, check your mail for my phone number.
Jack Snodgrass