Use a flosser that has a U curved bend, much easier to get in the back than regular string floss or usual flossers. Also breathe through your nose and out through your mouth slowly, I hear salt on the gums help too.
I'm sorry that I don't have a good answer for that. Flossing has been a habit of mine for years and I don't remember a time that I gagged while flossing.
I don't seem to have a problem with gagging while I'm flossing … I'm sorry if that's what happens for you. : ( I usually use floss picks, you could try that if you haven't already …
I prefer to use floss picks instead of string floss. Although my dentist told me string floss is ideal the floss picks are better than not flossing at all due to gagging. Another way to go is a water pick. My kid’s orthodontist highly recommends the water pick.