Commonly used in East London to show a feeling of sarcasm or frustration, like whatever to show you don't believe something or if you're getting told off and you're like whatever.
Mr. Teacher "Get out of my class! You've just got yourself a detention"
Boy "Gaang!"
Boy 1 "I just got on a N.A.S.T.Y mixtape"
Boy 2 "Gaang!"