Muriatic acid.
Use an old glass cleaner spray bottle. Shoot it on, let it sit for a minute and flush it out really good with the garden hose.
My wife's 94 GTX sat in the river from 94 until two years ago. I scrubbed a small patch for a half hour and barely changed the deep rust color to a lighter shade. I sprayed the acid on and watched the grime slide off the hull in less than five minutes. All I had to do was rinse with the hose, no elbow work needed. I hosed everywhere for a half hour to be extra safe.
As long as you only let it set for a few minutes it won't hurt the gelcoat. Just flush it off really good, don't let it set behind the trim, bumpers, anywhere.
I read marinas use a diluted solution to clean gelcoat hulls when they pull boats out for maintenance. I was skeptical but gave it a shot. It seriously removed over 20 years of river scum in minutes.