Are you looking for a functional repair or an aesthetic repair? Marine-tex epoxy is great stuff for filling/repairing holes and would be a great choice for a functional repair. It comes in white and grey and can be tinted with color. It's a two-part epoxy and I've found it pretty easy to work with. A little scar that's below the waterline is surprisingly easy to get past. A perfect gel coat match with no evidence of a repair is way beyond my skills and I'd have to take it to a fiberglass shop (but like auto body paint, you might have to redo the whole thing to make it perfect.)
2007 Mobius LSV
1989 Sanger Skier DX - sold