When something is totally wrong, gross, disgusting, out-of-whack, but taken to the extreme. Also, can be taking to the other extreme, in the positive sense.
That girl was ugly. She wasn't just gnarly, she was guh-narly.
That wave was killer, totally guh-narly.
The face you make during a song you really like, and you're dancing really slutty to. It probably involves pouting of the lips in an "o" like fashion, sticking your ass out and grinding like a cat in heat on the nearest person.
The girls have their slut faces on as they dance to "My Humps".