I am going through gallon in 3-4 days... milk has a lot of calories, so you better be seriously working out if you're eating big and drinking a gallon a day.
I'm on a strength training program, and along with a clean diet it has helped me put on 10+ lbs. I plan on slowing down the milk as I switch to cutting.
I did about half gallon a day for a month. I think about 90% of my gains were in my gut and I had to do a lot more work to get rid of the extra body fat. You will gain but just expect fat gains to come with it.
I did it back in high school to bulk up 25 lbs or so.. you will put on weight for sure but I would avoid milk altogether.. Maybe someone else can offer their .02, but once I cut milk out of my diet and started eating my calories - I started to make real progress. I put down 0% greek yogurt all the time but thats the only dairy I consume.
I used to drink near a gallon of milk a day when I was bulking and gained a ton of weight it's not lean mass a mix of fat and muscle but you do gain a lot especially on whole milk