How much does a gallon weigh

Weight measurements are the units conventionally used to calculate the weight of a body, that is, the amount of matter in it.
The standard measure of weight is the gram (g), taken from the decimal metric system along with its extensive list of multiples: Decagram (Dg), hectogram (Hg) and kilogram (Kg) which represent 10, 100 and 1000 grams respectively.

Other weight measurements: Quintal metric, Ton (1000 kilograms or one million grams), Pound, Ounce.

You say: The gallon is a unit of volume. For example, the huge jugs of milk sold usually weigh a gallon.

Please could you explain to me? I really can’t understand