A catchall term for heartburn, acid reflux, ulcers, and other things where your stomach feels like it's digesting itself. Since for the most part, you take the same pills for them, there's rarely a need to differentiate in casual conversation.
"Oh man, I've got some killer gut rot, anyone got some Rolaids?" "Here, take a Zantac, it'll work better. And maybe you should stop working 80 hour weeks and eating garbage?"