I advise that you look online and look for a gang map to find all the gangs in a contested turf. Destroying all those gangs first will help you in the long run when you are on missions. Some gangs only come out when your gang heat is really high, to raise it just kill a dealer or two on the street. Remember, you can't kill gangs on other turfs untill they are contested, until you move on to killing the guys from the previous turf, they will not be contested. It will appear when you enter the turf if they are contested.