A gallon is a unit of measurement for volume and liquid capacity.
Gallons are pretty much just used by Britain and the US.
Water weighs much more than gasoline, milk can weigh more than water.
Despite our apparent familiarity with it, it can be a little tricky to pinpoint the exact weight. This is for two reasons. First, a gallon of water will weigh more than a gallon of gasoline. Next, a gallon is a way of measuring volume not weight.
A gallon of water will weigh about 8.3 pounds in most instances.
Milk, for the most part, is pretty similar to water in weight. A gallon of milk will only weigh about 8.6 pounds. However, milk can have different fat contents. Skim milk and full cream milk are going to be very different in weight. Full cream milk can weigh as much as 11.6 pounds.
A gallon of gasoline weighs just 6 pounds. Far less than water.
Remember, a gallon is a measurement of volume not weight so it really depends on what it is you are measuring. NB:Water weighs much more than gasoline, milk can weigh more than water.