I use a children’s toothpaste. When it was bad, I would start with my top teeth, on the outside, then the outside of my bottom teeth, then the inside of my top teeth, then the inside of my bottom teeth.
I would use a non-electric toothbrush (used electric for the first time in a long time yesterday! But not taking it for granted!). As soon as I started to feel gaggy because I wasn’t tolerating the toothpaste, I would rinse out my mouth. I would then continue brushing for as long as I could readily tolerate, but no longer. Sometimes my teeth weren’t properly clean. I accepted that this would happen occasionally.
If it was all too much, I’d sit down for a bit with a glass of water. I wouldn’t resume cleaning after I started gagging.
Afterwards, I would rinse with Listerine children’s mouthwash. It’s milder in flavour (I have, on more than one occasion, been brought back from gagging by its flavour!), and formulated for people who aren’t very good at cleaning their teeth!
The toothpaste I use is 6-8+ (tried a few brands - I’ve attached a photo of one of the ones I like. Would avoid Oral B). Some people say a strawberry one is the best, but I can think of nothing worse! (Except maybe a peach one.)
Good luck! It really isn’t easy. You’ll probably find it gets better - mine is almost back to normal (except that I wouldn’t venture to use grown up toothpaste, or my usual favourite) - I’m 18 weeks now.