you need 2 different drill bits
Use the drill bit that you want the screw to set into. Drill through the gelcoat through the fiberglass. Then take a drill bit that is larger, and drill only through the gell coat so the screw can't bite into it. Whola, no gelcoat cracks, i have been doing this method forever and have no problems, just make sure to seal it up with a good sealant.