I see a lot of DIY repairs.
The main points where people go wrong is not grinding back to good solid material, not drying out thoroughly and not degreasing properly.
Preparation is more than half the job!
After that it's the choice of reinforcement and finishing.
If the damage only just extends through the gel you can fill with gel coat (with special wax added) and fair in with wet and dry. The alternative would be a good quality epoxy filler such as International watertite but the colour difference will be extreme. DO NOT USE CAR BODY FILLER!!!!
If the repair is deeper then you will be better getting it done at a marine composite specialist unless you have prior experience.
Chine repairs are an absolute pig even for the trade.
For anyone who is interested, here are some photos of a particularly awkward one we did a few months ago:
This is what the previous repair did after a few months.
After partly grinding out the cracks:
A repair piece ready to be inserted:
The repair piece epoxied in possition:
Clamps holding the additional epoxy/polyester reinfocement in place:
Fairing in the repair prior to applying an epoxy top coat: