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.
I've always wondered myself what happens. I imagine the Judge gets in some sort of trouble. But, yes, you probably would be dismissed from the military. It's called fraudulent enlistement. It is looked on unkindly.
Though there is the grey area where the judge might be more lenient if, say, the guilty were considering enlisting. Knowing that being convicted of say, car theft, would probably prohibt that, the judge and prosecutor might be willing to get together and change it to joyriding or something. Like I said, grey areas
Tattoos are now photographed and documented. They run them by law enforcement to check for gang affiliation. Other than that, there isn't much else a recruiter can do.