If there is only a gelcote void, you do not need to lay in new fiberglass and epoxy unless you have a huge gap to fill. I doubt you do, or you would have found this much sooner!. You can just grind out the void and cracked gelcote, clean it with acetone, and fill it with thickened epoxy or marine filler. You can then apply new gelcote. The idea is that the fiberglass layup trapped some air underneath and created a gap (void) between the fiberglass and the gelcote. I had dozens, maybe a hundred of these voids on my Telstar. Grinding them out, filling them, fairing them, and applying the finish, paint in my case, fixed them fine. I have only found one more after 2 seasons sailing. Now, I have created a few more spider cracks, but those are treated the same as a void, so no biggie.
If the fiberglass underneath is compromised, and you can see the core, you will need to see if the core is wet or rotted. If it is wet, dry it before repairing it. You may be able to aid drying with acetone. If it is rotted, you may be able to fill it with epoxy, or you may need to replace the core. There are techniques for both.
I second checking out Andy at Boatworks Today on YouTube.
