just go to your local boat chandlers or search on the web, there is a repair kit that we used in the boat building trade
it comes as a two pack one gel coat the other hardener. mix togeather as any other two pack and apply to a cleaned, dry, if possible scraped out damaged section and once it's dry and hard you can cut it back with 000 gauge wet and dry paper until you cannot see it any more.
I have done this on million pound sunseekers and the customer could not find the repair afterwards.