Barrier coat (usually an epoxy formulation) is required because gelcoat is not impermeable to water. Gel coat is not put over top of a barrier coat.
Boats that need it are taken back to bare glass, non-osmotic hulls can be taken to the point where no paint is left anywhere (back to 'good conditions' gelcoat), seriously osmotic hulls are 'peeled' - the gel coat removed entirely by a special mechanical plane-type tool. The hull is then sealed with the barrier and paint applied to that (after proper prep)
Boats without osmosis issues are often barrier coated as a precautionary measure. If you have NO osmosis whatsoever on a 40 yr old hull it's possible that it's actually already been barrier coated, if not you're not likely to have that problem.
In your case, fill and fair the deeper gouges, removing all the old layers of paint would be a good idea. After that, it's kind of your option.. simply repaint, or barrier coat for peace of mind. Time will tell which decision was the right one....
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)