When I graduated from high school almost 20 years ago, the idea of taking a year off before beginning college classes was anathema to my type-A, ambitious peers whose scholarships and degree-completion deadlines meant starting ASAP. The year of traveling, studying, or doing an internship abroad (typical activities of the "gap year") was considered the junior year at the four-year university and no earlier. I'd encountered the expression "gap year" in British TV shows prior to that time, but it was a strictly foreign term and idea in my locale (Pacfic Northwest).
For someone who made this transition just six years ago (in the US, I assume) to consider it a common term is astonishing to me. That suggests not only has the vocabulary term achieved saturation, but the practice is the norm, exemplifying a cultural shift as well.
What about "twat"?
If I may add another NOOB, how about the phrase "knock you up?" An acquaintance concluded an evening spent at one of London's finer drinking establishments by informing me that he would knock me up in the morning. My brain may have been a bit foggy at the time but I recognized immediately that he meant something very different than my initial thought given the physiological impossibility of that occurring!
I came to vote specifically because "gap year" was on the list - I had no idea this was a Britishism, it sounds that natural to me. Definitely heard it a lot when I was graduating high school/going to college 6 years ago, and didn't think twice about it.
I don't know that there is another term for "gap year" besides "gap year." As the parent of a child who recently started college (after taking a gap year) I can say that there are many U.S.-based conferences/books on the topic.