gallon noun Gallons. (used after a numeral or other quantifying word). [reflects plural partitive genitive case from Old English]

1939 Hall Coll (Emerts Cove TN) That way you get about, well an average of five gallon [of] whiskey to that if it turns out right.  1974 AOHP/ALC-807 We stayed over there with them till we drink it all up, drunk the six gallon or gave it away.  1997 Dante OHP-45 We’d get out in the summertime and pick blackberries. We’d sell a hundred gallon of blackberries.