A gap year means the odds are she won’t be going to college. Is she going to lazy around, or party hearty, or get a job to pay her way? Gap years are jokes.
Until now, that was a British custom.
They're becoming more common lately, particularly as adolescence is extended and adulthood pushed further and further away.
Bum around for a year or two before college. If you're well-heeled, go backpack across Europe. Go out and party, sow your wild oats, start college at 20 or so.
Might as well. It's not like there are awesome careers out there waiting for you to graduate.