Some people have a problem with the diaphragm call,
Making them gag.

I figured out what causes it.

If you put it to far back on your toung thats what causes it.
So when you start to gag just push it a little farther towards
The frount of of your toung.
That should fix the gagging problem.

I found this out while practiceing with mine.
I was trying different postions to get different sounds.
And when I had to to far back it gagged me.

So try not to stick it to far back on your toung.

Good Luck