[Ant]

Good advice already, first you need to remove any damaged material then bevel the edge of the hole/crack with a 12-to-1 chamfer, that is the area you scarf out will be approx 12 times the thickness of the glass, using a disc grinder or 36grit sanding wheel, then go through the process of laminating with the same material.

There is no easy way to do a less evasive job if the cracks are down to the actual reinforced glass or that is cracked through as well. The cracks will show back through the paint. Gelcoat has no real strength and is just there to cover the matt etc and for a painting base!
There are good fibreglass tips on Youtube or go to a reputable body or boat shop.