I bought this gallon a few days ago and went to pour a glass, but the jug is swollen tight like a drum, yet the date is barely halfway through. What would you do?

best to err on the side of caution. 

      7. Should You Return It, Toss It, or Call the Store?
If you find a swollen milk jug, it’s often best to return it to the store for a refund or exchange. Most retailers have policies in place for such situations, especially if the milk is within its sell-by date.
Contact the store, explain the situation, and inquire about their return policy. Many stores will offer a replacement or refund, as customer satisfaction and safety are priorities.