'G' is basically another coinage for “Gangster”. A gangster is a criminal who is a member of a gang.
Gangsters are also called mobsters or mafia, a term derived from mob and the suffix -ster. In this video, a man in his mid-70s gives tips on how to be a “G”; irrespective of the kind you want to be; either “A wannabe thug”, who are often illiterate or “the real gangsters”, who are those behind organized crime. His actions and facial expressions may be funny but he sure did give out some useful tips.
Although the exact origin of the word "mafia" is uncertain, some believe it originated in 1282 during the French invasion of Sicily and the saying, "Morte Alla Francia Italia Anela" (Death to the French is Italy's Cry), or MAFIA. Then, eventually the word "mafia" came to mean "manly", in Sicily.