The technique that I have HEARD about, but never actually done, is to drill a bunch of holes over the delam area. Put one or two molly bolts in strategic places. Then squirt epoxy into the holes between the delaminated sheets. Then tighten the molly bolts and wipe off excess epoxy. After it has dried, remove mollies anyway you can, and gelcoat.

That is it in a nutshell. Good luck.