I think the key is, no matter what they do in the gap year -- it needs to be near complete independence from the parents, or as much as will allow.
Working a crap job, while still living with your folks, doesn't teach much. It may offer them more disposable income to entertain themselves with, but not much else -- in my experience.
That's what the military did for son #1, and soon to do for son #2 -- remove the protective cocoon. Mom and Dad aren't there to save you, nor guide you. As close as you can get to tossing them into the deep end of the pool, but with some nice guardrails -- i.e Uncle Sam.
Son # 1 returned to me 40 lbs heavier, 100 times smarter, and 1,000 times more mature. He's done well.
Obviously not an option for everyone, but illustrates the point I'm intending to make.