Same as nub, but, the G letter is actualy a 1337 typo wich originaly was: fucking nub, but the spacebar was pressed a bit too early and it ended up: fuckin gnub. Since "fuckin" is a legit 1337 way to spell fucking many people mistake gnub for a totaly new word.
So in short - gnub = fucking nub
(verb) - to cause a mild, rather pleasant rippling sensation via the use of small, crucially timed yelps 36 cents below C# and placed on the abdomen of the person to be "gnuberated"
At the height of her pleasure, I gnuberated my wife and caused her to expel a sticky transluscent goo all over the coffee table.