I have never found any evidence that low calcium can damage the gelcoat in any way except for either higher incidence of staining or the formation of 'black spotting' or "cobalt spotting' which is actually cobalt crystallizing out of the gelcoat (gelcoat does contain colbalt). This look like little grey to black spot on the surface that start out pinhead size and grow into a 'crystal' that is raised from the surface. Newer gelcoat formulation are supposed to be more resistant to their formation.

I have seen firsthand that higher CH levels do help limit iron staining.