gang wars

To take over an enemy gang's territory, simply enter the territory on foot and start attacking gang members. Once you've killed three gang members, you will be notified that a gang war has begun. health and armor pickups spawn in the streets, and the first wave of gang members will move in on you.
Generally, an area with a darker shade of any gang colour would mean a greater presence of that gang in that area, and thus more gang members of that gang would be encountered when wandering around in that area.

This would also mean that some areas shaded light purple, for example, would have few Balla members roaming around and thus making it hard to trigger a gang war in that area. A small patch of land would also contain few enemy gang memebers. These can be overcome by walking over to individual or paired Balla members so that they follow you, then walk around with them chasing you until you find another mini group or a Balla car. You would then have three gang members that you can kill to trigger the war.

Another useful method is to leave 'highly concentrated' areas to be taken over last, and the 'less concentrated' areas first. This is because a gang war can also be triggered by shooting enemy gang members from afar. As long as you are standing in the area with little enemy gang presence, you can bring that area to war by killing gang members from the neighbouring areas. This sometimes cause a minor bug where killing Vagos members for example leads to a gang war with the Ballas.

There are a total of three waves of enemies you must defeat. They may attack unexpectantly or in large numbers, so make sure you have a plentiful supply of health and armor before starting the war. If possible, shoot from long distances or from rooftops. Enemy gang members will almost never walk on the roads unless it is absolutely necessary to reach the pavement you're on, so you can use this restriction to your advantage. Killing all 3 waves of enemy will win you the gang war and cause the territory to fall under Grove Street Families control, with Grove Street members beginning to wander the area (In larger or more concentrated areas, sometimes the last few enemy gang members would run away and the gang war would be won anyway'. Taking over all of the other gangs' turf in Los Santos will mean that the Grove Street Families have completely taken over Los Santos; the player's territories will no longer be attacked.