Gel coat products can be used as finishing for fiber-reinforced material. They can give the surfaces a nice gloss that makes old things look new. This is usually applied after a damaged spot is repaired. It goes on top of the resin mixture as the final touch for minor fixes.
The repair process requires the use of sandpaper to smooth out the resin that cover the holes and cracks. The original shine of the surface is lost but the gel coat can bring it back. Just follow the instructions and you should get excellent results.
Remember that different brands use various formulations. You need to follow their specific instructions to get a good outcome. Check the color on the can because it needs to match your bathtub or marine vessel. Most are available in bright white but you can add coloring to get your desired hue.