Ideally you want to move your uvula right out of the way and this should help with the gag reflex.
When you're going down on him, try doing it with your mouth completely open (rather than wrapping your lips around his shaft) and actually let the tip of your tongue sit outside of your mouth. It may not feel like the most attractive look in the world, but this will open up your throat and make a much larger void for his penis to enter.
Positions which keep your neck / throat straight also help as the head of his penis will go down your throat, as opposed to just banging against your uvula.
Also, don't forget to breath. Exhale through your nose as you slide down his penis and be concious of your breathing at all times. This will keep you relaxed which also helps with keeping your throat open and your uvula out of the way. It also takes your mind off of your gag reflex, allowing you to get past it. And it stops you from getting out of breath and tired meaning you can go for longer :)