What kind of gang violence? Murder? Witness intimidation?
Most juvenile criminal cases are handled by the state courts - very few juveniles end up in Federal custody, so the length of the sentence depends on the state where the crime was committed. (Google the phrase "Federal Juvenile Delinquency Act" for more information about juveniles charged as adults in Federal court.)
In California, a juvenile offender can be sentenced to state custody until his/her twenty-fifth birthday for a serious crime, but most of the time, they're out by age twenty-one. Juveniles can be charged as adults in California from the age of 14, but this option is reserved only for the most serious crimes, and most of the time, the little darlings are charged as juveniles.
You're looking for a crime for which the juvenile would be sentenced to two years or less. Again, it depends on the jurisdiction, but drug crimes, car theft, repeated burglaries, all come to mind.