What was found
FA IN AFT GALLEY AND AT 3R EXIT SMELLED HEAVY GASOLINE FUMES ON INITIAL PUSH AND ENGINE START-REPORTED SAME TO FLT DECK ARRANGED RETURN TO GATE FOR MAINT INSPECTION BOTH ENGINES RUNNING APU OFF FLAPS SET 1 AND ECON FLOW PUSHED FOR LOW PAX LOAD SMELL DISSIPATED QUICKLY NO SMELL ON RETURN TO GATE. FOUND FUEL COMING FROM HMU DRAIN. REPLACED HMU PER A321 AMM 73-21. RECONNECT DRAIN LINES. (LP 7633054, 7633060 - AIRCRAFT WAS GROUNDED)