Unlike OG (Original Gangster), which means a gangster who was around when the gang first came into existence, an RG is one who came into the gang after it first formed, and most likely was allowed to join becuase one of their OGs got a cap popped in his ass, and is now pushing up daisies.
RG = NOOB (for lack of a better word)
An RG usually doesn't have the skills nessasary and required to be a gangster. But they are working on becoming a real gangster.
Yo, ever since OG Clarence got popped in the ass, RG Fredrick took his place as the drive-by car driver.