The specific gravity of a substance is simply its density -- or mass divided by volume -- in relation to that of a reference substance, usually plain water at room temperature. The specific gravity of water is defined as 1.00, so a substance with a specific gravity of 1.50 is one and a half times as dense as water. Knowing the specific gravity of a substance, you can figure out how many pounds of it are in a gallon using standard conversions between units.
Gallons per Pound and the Conversion
A U.S. gallon is a unit of volume that includes exactly 231 square inches, 128 fluid ounces or 3.785 liters. One liter of water at room temperature by definition weighs 2.204 pounds, so one gallon of water weighs 2.204 * 3.785 = 8.343 pounds. Therefore, if you know the specific gravity of a substance, simply multiply it by 8.343 to get the number of pounds of that substance in a gallon.
The imperial, or U.K., gallon was originally defined as 10 pounds of water and is officially defined today as 4.546 liters. This is equal to (4.546/3.756) = 1.20 U.S. gallons. When dealing with the U.K. version of a gallon, multiply a substance's specific gravity by 10 to get the number of pounds it contains.