The gag reflex does serve a vital interest -- our longevity. Why anyone would want to surgically alter or eliminate something that is vital to our longevity simply defies logic. It certainly doesn't rise to the level of medical necessity when it is for the purpose of deep throating someone's cock or any other foreign object.
I'll leave the anatomy lecture to Dr. Danny who is well-qualified to speak to the issue if surgery is medically sound and indicated to enable someone to overcome the gag reflex in deepthroating cock or any other foreign object.
It seems to me that this kind of surgery would fall into the category of procedures for which there is no sound medical necessity. If it does and I'm wrong, I want to know where I can go and get a penis transplant from someone dying or willing to donate at least 12 inches.