Sand the additional area out to the tape line with 320-grit paper. This will be the total area to be gelcoated. Tint the batch of gelcoat to match the color of the boat. If the boat is fairly new and the manufacturer is still in business, you may be able to get gelcoat that will be a very close color match. If this is not an option, you will need to get the gelcoat from a FRP product supplier.
Many marine distributors handle the gelcoats from various resin manufacturers.
You will also need pigments to tint the base color to obtain a good match. These pigments are generally available from the gelcoat suppliers. Frequently, local repair yards that do fiberglass repairs will sell these products to the project user. There are several gelcoat additives that, when used properly, make matching a repair easier. These are clear, low-viscosity resins which are not air inhibited.