In Los Santos you will find several red zones. As soon as you get close, enemies can start attacking you, and if you fight back, a gang attack starts.
The principle ? Very simple: kill all enemies in the area.
Make sure you pack some protection and snacks though, you can quickly get overwhelmed. Especially since the enemies will be armed according to your rank. In other words, the higher your rank, the more heavily armed enemies will be.
From time to time, supply crates appear, containing ammunition, snacks, as well as a few dollars.
And as a reward, a little RP: 500 base, but an amount that can increase depending on the number of headshots you get.
Here is the map with the locations of in-game gang attacks. Note that gang attacks have a schedule where they are available.
Your turn now !