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Gas tank was overfilled. It overflowed and it and deck were stained. Exposure to light has helped but I'm wondering if there's another, more complete solution.
You might be up the proverbial creek but as it is a petroleum product that impregnated a porous material all you can try is to neutralize the petroleum.
You don't mention the tank and deck material.
Maybe straight alcohol and much scrubbing with a dish soap (degreaser) like Dawn or Simple Green.