there is even a 3rd one - from wikipedia:
The imperial gallon
The imperial (UK) gallon, defined as 4.54609 L, is used in some Commonwealth countries and was originally based on the volume of 10 pounds of water at 62 °F. (A US liquid gallon of water weighs about 8.33 pounds at the same temperature.) The imperial fluid ounce is defined as 1?160 of an imperial gallon.
The US liquid gallon
This gallon is defined as 231 cubic inches,[2] and is equal to exactly 3.785411784 litres or about 0.13368 cubic feet. This is the most common definition of a gallon in the United States. The US fluid ounce is defined as 1?128 of a US liquid gallon.
The US dry gallon
This gallon is one-eighth of a US Winchester bushel of 2150.42 cubic inches, thus it is equal to exactly 268.8025 cubic inches or 4.40488377086 L. The US dry gallon is less commonly used, and is not listed in the relevant statute, which jumps from the dry quart to the peck.[2]