Unintentionally left an unopened gallon of whole milk in my car for a few days.
What's a creative way to use it?

Make farmers cheese.

https://www.myrecipes.com/extracrispy/use-sour-milk-to-make-cheese

Yep make farmers cheese. The mix it with mashed russet potatoes, and make pierogi. That is the classic pierogi.

That's rotten milk not sour. Pitch it.

Depends. Slavyanka leaves her fresh milk in a container at room temp for 3-5 days then makes farmers cheese. Starts to get thick like Kafir.

It’s not any good when it’s good. Sure wouldn’t be better when it’s bad.

Send it to a military school - that'll cure it of being spoiled.

when I was on the farm the pigs loved it..

Bet they loved buttermilk, too.