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What is your goal?
If you want to just use your boat,
grind the gelcoat and paint about 2-3" around the gash,
cut any loose fibers out of the hole area to the foam.
Coat the area with resin to seal.
Fill the hole with small layers of FG,
or a chopped up FG/resin mix,
or a spread of bondo if it's really deep,
then a couple patch size layers.

Now grind/sand it down flat,
depending on your level of cosmetics.
The on the water performance will not change if it is raised a little,
so smoothing with filler before paint is your call.

If you want a winter project,
than flip it over and start sanding!

TP