The amount of pounds in 1 gallon depends on what is in that gallon because water, oil, milk etc has different densities and are not the exact same weight in gallons.
But for a gallon of milk and water the gallon of milk or water weighs in around 8 lbs but can be as much as 8.3 lbs.
A gallon of oil may only weigh 8 lbs while the gallon of water and milk weighs a few ounces over the 8 lbs.
So to find out exactly what the gallon of liquid you have weighs you can place the gallon of liquid onto a bathroom scale or other weight scale to get an accurate weight.
But the gallon of liquid should weigh in around at least 8 lbs but can be a bit more or less depending on the density of the liquid.