I reckon I'd dust off the old Googler and have a look at gelcoat repairs to get a feel for what might be involved.
For what it's worth, and bearing in mind the size and type of injury you're treating, I'd probably have a bash at that myself.
If it was bigger, or had crushed or gashed the surrounding GRP I'd be taking a phone full of photographs (including something for scale) to a boatyard for advice, but that looks like a relatively straightforward puncture wound.
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