Are you brushing the toothpaste into your tongue all the way back as far as you can?
When I first started I would gag. But the more I did it, the further back I could brush without gagging. And now my mouth doesn't feel clean unless I brush my tongue too, all the way to the back of my throat.
Excuse me if this is preaching to the choir and you are already doing it. But a lot of people don't. I ask them if they brush their tongue and they tell me no, because it makes them gag. It makes everybody gag at first. You have to get used to it.
I had a friend with chronic bad breath and his turned out to be a tumor in his lung. It was an easy to cure tumor and they shrank it to nothing with chemo. He is completely cured.