I see two possibilities... Repair, or replacement of the cracked gelcoat area. It depends on what you want, and it also depends on whether the cracked area you're talking about is small (say, a one-inch square area) or larger.
If the layup is a dry one on the inside, and you can reach the inside, you might try poking a few hole in the fiberglass cloth with a pin, but (if possible) not poking through the exterior gelcoat. Then, using a gloved hand, or your finger wrapped in cellophane, you could apply some medium CA to the interior, rubbing it in, so that it goes through the holes you poked, and allows you to press the gelcoat and fiberglass back together.
If that works, then you're just left with the exterior repair. You don't have to use gelcoat. (available from Fiberglast, Inc. at
http://www.fibreglast.com and other composite dealers ) You could use ordinary polyester resin, gently brushed into the cracks, sanded, primed, etc..
If you actually want to fully restore the part, then skip the CA step, sand through all the area of gelcoat that's cracked, scuff the area, re-spray the gelcoat, sand until happy, etc.. It's a more tedious process.