How to control gag reflex?

First idea is not to take it too deep.
One idea I gave a GF years ago is to take my cock deep enough without gagging, grab the shaft placing your hand against your lips, lift your lips higher and move your hand a bit higher again. That said, don't tug the shaft skin before doing that. Let the skin at its natural position.
If you get lost in the moment you know not to go deeper than your hand acting like a stopper. And holding the shaft does two things. For me anyway.
It feels good, and it's a visual stimulation to see a woman hold my cock.

 

Sure, go for it.

 

I want to be your guest!