Here in the U.S.A. there are prison gangs.

Answer

Sin is not kufr regardless of how great and major it may be. However, if the sin is regarded to be permissible, it will be kufr. If a man steals or fornicates and believes that it is permissible, then he becomes a kaafir. But as long as he understands and accepts that his misdeed is sinful, he remains a Muslim.

If the gangster believes that his acts of drugs, robbery, etc. are haraam, he remains a Muslim, albeit of very weak Imaan. If he believes that his misdeeds are valid and permissible, then he leaves the fold of Islam. If the gang has any beliefs of kufr or some code which is kufr to which the gangster has to swear allegiance, then such beliefs will render him a kaafir.