Hiya Michel and to the forums.

Not an expert either, but I agree with what Maddie said - it's about something hitting the back of your throat that triggers the reflex. For me it's even just the thought of something big and opening my mouth accordingly that sometimes makes me gag...like a dentist's hand, lol...not only a gag reflex, but a bite reflex there.

From what I've heard and read here it can be trained, though...trained to control it, that is. Breathing evenly through the nose, trying to relax while training with your gag a certain time each day should help. Then again, I hope some of the experts here will give you more detailed advice that that.