Extremely powerfulinsult usually directed towards someone when a more creative term than 'asshole' is needed. The implication is that the person is so amoral/stupid that they would fuck a gunt as opposed to a cunt. Reasons for this lack of proper copulation technique could range from indifference to desperation to a perverted sense of the need to humiliate oneself and sexual partner simultaneously.
Person A: Did you hear about Ronnie? He date-raped that retarded girl on Pearson Avenue.
Person B: Mikey's sister?!? That guntfucker! I'm gonna kill him.
When a man possesses such a freakishly large penis that he ruptures his partner's abdominal cavity during intercourse, resulting in penetration of the intestines.
"Being hung like a horse ain't all it's cracked up to be. I accidentally gave my girlfriend a gutsfuck the other night, and now she's in the hospital."