Rust stains on Gelcoat are the bane of a boaties existence.
Every bit of stainless steel wants to mark your gelcoat and nothing seems to remove it. But there is hope.
Rust, unlike Gelcoat has a very specific chemical make up that contains oxides of iron and as such can be targeted by good quality cleaners using Oxalic acid wash, with a pH of 1.3.
Whereas the acid will not react with a neutral gelcoat and is safe to use it will react the iron oxide and remove the rust quite easily.
You can use good quality cleaning products targeted at rust or just get something like “barkeepers friend” which is a powered version of Oxalic acid.