Probably a good idea if we are heading for war. At least a few more people to know what to do if the worst happens.
The problem as I"ve heard it expressed over the years is that, one or two roles aside, you can't teach much that is worthwhile in a year.
If you make it a two year "gap" with some sort of reserve obligation with a recurrent training commitment thrown in for x years afterwards then it might be cost effective
If however the idea of this gap year proposal is to give people a short taster of multiple aspects of the military, with them hopping around from base to base they won't really learn anything and will be a drain on resouces.
As a result of that thinking a lot of national service around the world, certainly in Europe these days, is non-military,