A gallon is a standard unit of measurement used to measure liquids, while a pound measures mass; it is not possible to convert between the two because they measure two different things. To calculate the weight of a gallon of something, the gallon should be measured out and weighed on a scale.
The weight of a gallon of a substance varies based on the weight of the substance. For example, honey has a higher density and is heavier than water. Even though a gallon is the same in terms of the amount of liquid, the weights would vary.
The density, or heaviness, of water varies based on temperature. A gallon of water at 32 degrees Fahrenheit weighs 62.416 pounds, while the same gallon weighs 60.135 pounds at 200 degrees Fahrenheit.