People naturally have differing levels of gag reflex. While I haven't tried swallowing a cock (yet), I've learned from ENT visits that I have a "moderate" gag reflex. Unfortunately, my wife has a severe gag reflex

That said, gag reflex can be lessened --and in some cases eliminated-- by focus, relaxation and practice. If you do a google search on it, you can even find training plans (popsicles apparently are a good instrument for practicing to relax your throat, if you don't have something better on hand). There are also tricks, like squeezing your left thumb --no one knows why this works, but it does in fact work for a small percentage of people-- or using a numbing spray (although that's dangerous IMO).