As Rob said Gelcoat is a resin based layer with pigment added for color. It's used a lot in construction with female molds. Its layed in as the first layer and when the part is removed it has a perfect paint like finish avoiding a lot of final finish work(cost).
Building without a female mold requires spraying it on. It must be cut with a thinner to push thru a spayer. The thinner evaporates during drying and leaves an orange peel like surface which requires a lot of sanding and buffing to return to a paint quality finish.
Another problem is repair. Gelcoat ages and slightly fades with time so that you can't exactly match the color
Bill