The tank appears to be one of the replacement tanks from the 80s. The give-away is the welded edges instead of the rolled edges of a factory gas tank. I'll check my spare one and see if the same gunk is on the bottom. It may be there to help seal the drain plug area as well as some of the welded seams. It is possible this was put in by the manufacturer and was never intended to see alcohol.
IIRC there is a baffle in these tanks which would make scraping half the tank impossible, but as suggested by the others, heat or solvents may remove the gunk.
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