I would personally be aprehensive about drinking 1 gallon a day..
I can't remember the article but I remember reading a study done on the correlation between intaking high dosages of milk and developing autism. Humans are the only species on the plant that continue to drink milk after being weaned. It is a source of nutrition for a new born baby not a athlete trying to put on weight.
I havent done extensive reasearch into the topic but i believe there are better ways to gain nutrition than drinking a gallon of milk a day. If you wanted to gain weight artificially why not try steriods?? it could be just as bad for you and you could get better results..
my 2cents would say take in a higher percentage of protein and fats in your diet from quality foods not just from a gallon of milk