I use toilet bowl type cleaners to take tough stains off of fiberglass.  I doesn't ruin the gelcoat and 30+ yrs of doing it on my boats has never damaged anything.  The acids have changed throughout the yrs but these products still work. Do a couple of quick applications with rinses to get the idea how strong it is.   Rinse well and don't leave it on more than a few minutes between applications.

That's my 02.