It's how a rapper dances on stage and/or movie videos.
To do the Gangsta Shuffle, you have your thumb and forefinger out (or all fingers out but closed together) and you move your shoulders back and forth while sometimes pointing to your crotch.
"Hey yoooo, have you seen Tyrone?"
"Nah, but I heard he was in the club, waving his glock and screaming BIIIIGGGGG DICKSSS TO DA FRONT! That nigga is a gangstafaggot!"