There though is a very big difference in the amount of pigment in gelcoat and what you'd add to epoxy to make it white. The epoxy will tend to self level even with the pigment added, gelcoat never will due to the amount of pigment in it.

If the damage is in a place that tends to get damaged, a mix of pigment and cabosil added to epoxy to make it white, will probably last as long as a patch of whitened epoxy with a dab of gelcoat over it. There are likely worse things attacking that spot than UV.

People seem to have a compulsion with their glass kayaks that whenever a little bit of gelcoat gets messed up they want to fix it with more gelcoat. Chances are you don't need to do anything, but epoxy with silica (or even bread flour) in it will likely hold up better than gelcoat. Add a little pigment to it for cosmetic purposes if desired, but too much pigment will just make it weaker. Gelcoat doesn't "fix" any problems, it just hides them.