It isgreat stuff!
We used to use it on ships to remove brown rust stains from white paintwork, and it works very well, however it does discolour green paint if you leave it on there, it sort of fades it, same for red. I would suggest trying it on a very small descrete area before "splashing it on all over"!
However for gelcoat I am sure it would be fine.
A word of warning, also keep it away from any natural fibres, such as hemp/manilla ropes, or normal canvas, as it will rot them very easily. This was the reason we were banned from using it on ships. Since then we have tried dozens of different things, but nothing as good as Oxalic acid.