A GG is a short term that stands for Ghetto Guy or Ghetto Girl. Usually referring to a Ghetto girl. This is a fairly new term that originated just 3 or 4 years ago. The great thing about it, is that you can use it and the GG in question has absolutely no idea what you're talking about! They never catch on to it.
Sarah: Omg Tina look at those GG's dancing over there!!
Tina: LMFAO! omg. why would they do that..in public..?
Sarah: It's a GG thing. we'll never understand.
GG #1: OOOOO GURRRRL! YOU NEEDA BACK UP OFF MAH MAN FO' IT GETS UGLY UP IN HURRRRRR.
GG #2: WHY YOU SAYING I NEEDA BACK UP WHEN HE DONT EVEN WANT CHU NO MO?!