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.
Brushing your tongue is important though.
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.
Keep trying until you don't. That's how I learned how to swallow cock. You could also specialize in servicing micropenis if that sounds like too much work. 🤷🏻♂️
I don't discriminate lol
For most people it is a matter of training.
Yeah, some people have a weaker gag reflex than others, so it's easier for them, but at the end of the day most people need to train the ability to swallow.
The basics are: Slowly going deeper, bigger and lasting longer. Allow your body to adapt to the feeling without instantly "panicking" (= gagging). It takes time. The key is to relax, not to tense up.
If you tested your ability with a toothbrush already, you can make that a routine. Do it whenever you brush your teeth and slowly get used to the feeling.
Once you got somewhat used to that, you can move on to bigger stuff and try to take it deeper. I advise against trying to "force" it and go big too quickly, no matter how enticing it may be. You may end up throwing up.
As a safety precaution make sure that whatever you train with has something you can firmly grab onto or a flared base in the case of a dildo.
Any recommendations for the gradual increases?
Ultimately it all comes down to how comfortable you feel and how quickly your body adapts. It is an individual thing.
In regards to depth: Simply go as far as you can before you start to gag. Over time you will probably be able to put whatever you use to train in deeper before it becomes too uncomfortable.
You may simply have to test that.
The toothbrush is a good start. Alternatively to the toothbrush many people suggest opening your mouth as wide as possible - as if you are yawning - and then stick a finger in there as deep as you can.
Some people recommend moving up to a (peeled!) banana: It has the upside that it's soft, curved and size-wise closer to a dick. Once you can swallow that, you can move on to a dildo if you need to. Make sure that the dildo is on the softer side, because hard dildos can get you a sore throat.
Take your time with this, but keep at it. Most people do it once per day. Once again, it varies how long it takes, so don't feel frustrated iftakes a while before you will be comfortable with it.