Gelcoat is very brittle when drilling, if you don't do it right the edges of the hole will chip while you're drilling, and start spider cracks that will propagate a long way over time.
I was very successful with a step drill
, as Aric suggests. Use a small drill, run it in reverse until you get through the gelcoat, then forward through the fiberglass. Then use the step drill up to the diameter you need.