No.1 son had one two years ago. Went to Australia and, apart from several visits to relatives (grandparents, uncles, aunts, cousins) seemed to spend most of his time partying. To be fair, he had worked as a lifeguard and pool attendent for several months before he went, so had earned the money before he went. He managed 5 weeks work in a Brisbane coffee bar.
I figure that he didn't make the most of it, and certainly didn't do anything like what I did 30 years ago, but then he is not me.
No.2 son can't afford to have one as the difference in costs if he goes to Uni next year are so much greater than if he goes this year.
your DS1's gap year sounds like it was great 4merlyknownasSHD!
The Gap Years that turn out to be Worth It are usually the ones where the DC has something in particular in mind that they want to do. They go off, try to do whatever it is, and often discover it was harder than they thought. Even if it didn't work out as planned, they have a great time trying and mature a lot. If it's your/our idea then it won't develop the same independence or sense of achievement.
So I would say the actual idea needs to come from your DD, even if you plant the seed
My son took one - but when his school had its 6th form open day they were very clear it should have a purpose and the the next step in hand. Otherwise 1 year can become 2 or even 3 if said child not sure what todo. My son had German girlfriend - her parents recommended a language school attached to Dusseldorf uni. It also had the advantage of the only German uni with a cricket club - allbeit run by Newzealanders. Living abroad made him quite self relient. He's kept it going and is almost fluent. l'ed reccomend it