Head down to bias boating and buy high strength epoxy filler+ hardener compound(sold individually in cans 1KG and 500GMS,product name is norfill.Also buy white brush flowcoat comes in 1L can and small bottle of hardener to suit.(coulor additives needed if gel coat isn,t bright white to change pigment of flowcoat )
Mix epoxy/hardener together fill hole and leave to dry.
Sand repair to shape required,Now mix flow coat/hardener and brush over repair to suit,can thin product down with acetone if needed before brushing over repair.
Once dry approx 24hrs sand repair lightly with 400 wet and dry sand paper in soapy water followed by a sand using 1200 sand paper.
Finally buff repair by hand using t cut polish bought from super cheaps for a full gloss finish.
Hope this helps with the repair
chris.