Another good product that is safe to use on gelcoat, etc. is "RustAid - Outdoor" from Home Depot or Lowes. I primarily use it to remove rust stains caused by irrigation sprinklers around my yard, but have also been using it for many years to clean the brown stains off the gelcoat, etc. of my powerboats and previous white hulled Hobie14T caused by exposure to organic acids as: tannic, humic and fulvic in our brackish water (in addition to rust stains).
The active ingredients in RustAid are: Hydrofluoric Acid and Oxalic Acid. I usually dilute it with water in a hand pump sprayer, spray it on, wait a few minutes and then rinse it off and the stain is gone.
Once you get rid of your gelcoat stains, it is best to seal the porous gelcoat with a wax so it will be less likely to stain again.
