Hello passionseeker: I don't have any experience with this particular symptom. Assuming that there's nothing physically wrong, then I would think seeing a therapist might be the best thing to do.

It could be this is related to stress & anxiety. To the extent this is the case, anything that is useful for decreasing stress & anxiety could, potentially, be of value with regard to this gag reflex you're struggling with. Such things might include: exercise, meditation, medications, etc. I'm sure you've heard all of this before.

I've certainly had my own bevy of anxiety-related symptoms over the years. For example, there was a period of time when my eyes would flutter up-&-down repeatedly & uncontrollably. I didn't do anything about it. After a while, it just went away.