Being aroused generally reduces this reflex. I think I trained it a long time ago by rubbing the roof of my mouth with a (very clean) finger. Eventually I could play with my uvula. I cemented this skill by years later, training with a dildo.
Interesting, toothbrushes still bother me, and especially brushing around my tongue is a huge trigger. I don't brush my tongue and definitely not under my tongue.
Glad I said something. My dentists assistant told me years ago not to bother brushing my tongue, but to do that gentle scrubbing on the roof of my mouth. This is... probably just plain wrong, lol. But tbf I don't have tongue inflammation or cavities.