So you get dismissed from the Military and go to jail
The conditions are generally set that unless you complete your military service in good standing, you go to jail. It's a win-win really. That said, a lot of gang bangers are sending members into the service to learn combat strategy and what not. It's becoming far more common than anyone would have suspected. There's Blood, Crip, MS13, Latin King etc... gang signs showing up in Afganistan, Iraq and other countries where our servicemen are located. The military apparently has had to add that as one of there background checks. You probably know more about that than us Slate.
Marine here, did 6 years and a pump to that Sandy Place where people give you dirty looks.
With regards to "gang problems" in the service, while it is prevalent, alot of the the "gang tagging" and everything else you see are just from wannabe punks that try and regale everyone with tales of how hard they had it back in the hood before joining. Everyone I've met in the Marine's turned down a D1 scholarship to join, or used to be the hardest gang banger on the block, or was about to sign a record deal when their band broke up so they joined. Some of the stories you hear are hilarious.
I'm more concerned about the sheer volume of large and intricate phallic objects drawn in porta potties by bored service members all across third world countries than I am about a little gang grafitti.