The Gang wars start when you complete the mission "Doberman" You then have to attack other gang territories to make them your own.

But occasionally, other gangs will attack your territories. And you have to get there quickly to defend them.

But if you drive around on a police motorbike, and your territory gets attacked. Just start the vigilante mission by pressing R3, then exit it. And the territory will remain yours.

This saves a lot of time so you won't have to keep going back to defend them.