yeah in all honesty thats pretty dumb and not needed
way to much dairy can cause problems for some people
the amount of saturated fat
and there is roughly 2.5k calories in a gallon of milk.
your normaly diet is probaly 3500 ish calories
having around 6k calories a day at your age without being 150kg of muscle or running alot of drugs is just asking to become fat and unhealthy which in turn leads to alot of other issues.

Like post above said find BMR and eat 300-500 above it. muscle takes time to grow being in 100 surplus and 1000 is no difference but fat gain.