The Gall Chall is a shorter name for the gallon challenge. Its a much better name and easier to pronounce.
The Gall Chall itself is a competition in which competitors must all chug a gallon of milk, and the last person to vomit wins.
If no one can finish the gallon without vomiting, the the person with the least milk remaining wins.
Person 1 - Why is there vomit everywhere?
Person 2 - Oh, some dumbass kids were doing the Gall Chall.