A gallon of milk weighs around 8 and 1/2 lbs which can be a little bit more or lighter depending on the type of the milk.
Yes whole milk and Light Skim Milk weighs different and the skim milk weighs a bit less than whole milk but the average gallon of milk weighs in at 8 to 8 and half lbs.
You can put a gallon of milk on a bathroom scale to find out for sure what the weight of your gallon of milk is but a gallon of milk isn't too heavy and weighs just a bit more than some newborn babies weigh.