If you have heavy orange rust stains on your gelcoat then Oxalic Acid is good for removing them.
I cannot reccommend mixing concentrations, however you should find the relevant information on the box of crystals.
Be aware that Oxalic Acid is highly toxic – to humans and to fish. Even in low concentrations this harmful chemical kills fish. Use it on the spots and try to avoid contamination with water. There is a product called Y10 which has the active ingredient within a paste to avoid spilliage – this may suffice in most cases.