Another option is a very fine wet/dry sandpaper, used wetted with water. Say, start with 1500 grit and "work down" to 1000 or so to see if it removes the stain (but I would not go below around 600 probably because you are also removing a sliver of gelcoat as you go).
I know that normally one "works up" through the grits when actually using it to sand/smooth things, but in this case I figure it'd be better to start "high" and only go down as far as necessary. Then once the stain is gone, if you've gone down to 1000 or lower, you could work your way back up again.
Of course best to try in the ol' "inconspicuous area," if possible (of course since when do stains appear in inconspicuous areas :amgry)
Sunbeam :hot
PS: Interesting that FSR didn't work (if that's the gel remover you tried). Years ago, I had some organic stains (orange from some sort of one-time pine shedding that was not the normal type) that Would Not come out of white gelcoat, no matter what I tried - until I tried FSR. (That said, I think wet sanding would also worked but I didn't think of it until after I used the FSR.)
