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It's kind of hard to see what you're talking about in the photo but in any case, you could sand away the gelcoat, roughen up the surface and start filling with layers of fiberglass mat, starting with small circles, ending with a piece just below the rim of the depression. Sand it fair then fill with a mixture of resin and microbaloons and fair that in to fill any depressions left then reapply the gelcoat. Sounds easy and it isn't. We're it me I would either live with it or find a glass man for an estimate.