According to the USDA, a gallon of milk weighs about 8.6 pounds. A quart of milk weighs about 2.15 pounds.
Of course, since milk is an organic product, those figures are averages rather than exact figures. Depending on its fat and nutrient content, milk's weight can vary a small amount. For reference, the weight of a gallon of water is 8.34 pounds, and the density of water is about 1 kilogram per liter. Since milk is between 80 and 90 percent water, even an outlier gallon sample of milk only varies a few ounces from the USDA figure. Also, since most commercially sold milk is blended and homogenized, the chance for significant variation is even smaller.