1.00 US gallon of Water (68 F / 20 C, room temp) weighs 8.326 lb.
Remember that a gallon measures volume (space), while a pound measures mass. The conversion always depends on density, which changes with temperature.
Next stepIf your liquid is not listed, switch to "Custom substance" and enter the density from the product data sheet (SDS/TDS). Density is typically listed in lb/gal or kg/L.