It turns out that there is a traitor amongst the Grove Street Families, and Sweet wants to flush him out to put an end to the traitor. Upon starting the mission from in front of the Ammu-Nation, you will be given a quick overview of how to proceed. Basically, Doberman is a tutorial to the Gang Warfare aspect of the game. Looking at your map, you will see a purple highlight over Glen Park. This means that area is under control of the Ballas and you will need to exact some control over it yourself.
If you need any weapons, head inside the Ammu-Nation and purchase some, noting that your selection isn’t all too robust right now. Should you have done all of the Gang Tags, then it’s just easier to go back home on Grove Street to grab those weapons inside your house. The AK-47, which you can’t purchase right now, but can have from the Gang Tags, is a very powerful and useful weapon for taking over enemy gang territories. To begin a gang war, head to the territory in question and make sure you’re in the highlighted area.
While in it, look for the enemy gang, which will be Ballas in this case, so look for the NPCs with purple clothing, as that’s their color. Kill three of the Ballas and the gang war will be started, as indicated by the message on the bottom of the screen, as well as the area flashing red on your map. At this point, you will need to brace for three waves of enemies, with each wave getting slightly more powerful weapons. While doing these gang wars, you will find that there will be health and armor pickups that spawn in the streets, so always keep an eye out for those if you’re running low.
Upon clearing all three waves, the territory will be marked green on your map, meaning it is owned by the Grove Street Families and your own gang members will begin spawning there. In the normal game, outside of this mission, your own territories will periodically come under attack from enemy gangs. The game will warn you when this happens, and you need to defeat a single wave of enemies to defend the territory. For taking over territories, you will gain respect and money will spawn from a static pickup outside of your house on Grove Street, with it being replenished over time.
The mission isn’t quite over after taking over the territory, though. The traitor will come out of hiding when all is said and done, so run him down and kill him to finish the mission. After finishing the mission, you will see a bunch more gang territories will appear on your map. The same rules as described above will ring true for these, but it’s important to note that enemies can only attempt to take over your own areas if their areas are adjacent to yours.