It's quite simple, you need to practice, you're not born with the synaptic connections to willfully control that response, you need to build them up, first step is just becoming aware of what's firing when you gag, consciously mapping the feeling, then you practice resisting it. It's virtually unheard of for someone to be born without the reflex, but plenty of people learn to suppress it. Took me a few months to get it down to almost perfect. I can throat a few inches past the back of my tongue... although it's been more than a decade since i last had anything down there... maybe i'd be rusty now, not sure how much like riding a bike it is...