You can use Kreem the original one from Germany, works too. Is resistand at least to our E5 and E10 fuel.
(Don't use the "Kreem Complete Petrol/Gas Fuel Sealer Kit" which comes from the US, this is a plagiarism product witch has a lousy quality and is a lot cheaper too.)
And you will use the cleaner first which is very agressive and will "dissolve" the surface of the resin so the surface got rough, as rougher the surface as better the sealant will connect with the resin of the fiberglas. People that are complaining of chipping paint often haven't roughen the surface enough, a real task with gas tanks made of fiberglas. Used it for my glasfiber tank too, no problem so far ... (pic possible if desired, (warning a real gleamy item))