I recommend always using a countersink before drilling anything but the smallest diameter hole through or into GRP. This in effect safely grinds away a circle which you can make the right diameter for the actual hole that you want to drill (in fact just slightly oversized is best).
If you go straight in with a drill anything more than about 2-3mm diameter, the flutes on the drill bit tend to ‘grab’ the gelcoat and break chunks of it off before you have time to stop the drill.
If you can be bothered (and why not?), drill a pilot of say 2mm diameter to guide the countersink, then use the countersink, then drill the intended hole(s), including the clearance hole mentioned by Colin, if applicable, for which you can now safely use a normal drill bit, having already created an adequate countersunk hole.
In an emergency you can get something like the same effect by using your drill in reverse gear, i.e. with a (full-sized) drill bit rotating anticlockwise.