Is it the taste of the toothpaste or having the toothbrush in your mouth that causes it? How long has this been going on? Does this happen with anything else, like if you leave a spoon or fork in your mouth too long? What about gum or mints (or anything that you have to hold in your mouth without chewing).

I feel a gag reflex if I brush too far back or hold too much tooth paste/spit in my mouth without spitting, so maybe yours has become an automatic reaction. You associate brushing your teeth with gagging.

I would suggest brushing slowly and taking deep breaths through your nose while you do. Start at the front of your mouth and slowly progress. When you feel you're about to gag, breathe, take the tooth brush out, and spit. Then resume. Also, use a small toothbrush, even child's size if you have to (or an electric one with a small head) and a small amount of tooth paste to start. If you think it might be an association with the taste of the toothpaste, try switching to another flavor, like cinnamon.

You could also try distracting yourself. Play music or watch tv while you brush. I've also heard that numbing throat sprays (like for sore throats) can calm the gag reflex.

If this persists or happens at other times, you may want to see your doctor. It could be acid reflux or your tonsils/adenoids.

this happens to me, as well. it's not every day, and not with all toothpastes, but i definitely go through phases when it happens every time i brush- morning or evening. like the first writer, i've also been celibate for many years, and pregnancy isn't the cause.
None of the above suggestions would have any bearing on my situation, as it's not a reaction merely to the taste or to the size of the brush. it starts
For me it happens:
-every time i use children's bubblegum flavored toothpaste.
-other bubblegum flavored things do not cause it.
-other flavors of toothpaste DO cause it.
-more often with gel than with paste.
-begins the moment the toothpaste touches my tongue. begins with VERY heavy salivating, and a nausea. -rinsing my mouth doesn't stop it.
-breathing deeply while bent over helps, but doesn't stop it. (salivating the whole time)
-if i do begin to retch, it's a tossup as to whether or not it ends in vomiting.
-in bad cases, my gag reflex will keep up for half an hour. in mild ones, only a couple minutes.
-i've found that quickly gargling with listerine helps to short-circuit the process, if i catch it before the gagging.
the reaction happens whether or not i'm menstruating or at any particular point in cycle.

i'm not on other medications or physical/mental health issues.

I have never come across ANYthing other than toothpaste which trigger this reaction.

Could it be an ingredient in the toothpaste that is causing it? Like fluoride or something? You say it's not with every tooth paste--which ones don't cause it? Have you tried any suggestions I wrote? Have they helped?