Here's a map showing all the GTA 5 Online gang war / attack locations and times they are active. There are around 52 gang attacks on the map and you can make around $2000 per attack picking up crates and cash / weapons found on the floor around the gang attack area. ($104,000 total) In addition to cash, you also get RP, ammo, and weapons.
(Click the image below for the full size.)
Here is what the crates look like, they can contain $500, weapons, or ammo.
Thanks to tech for the map.
Other GTA Online Pages:
- GTA Online Hints, Tips & Tricks
- Making Money in GTA Online
- Los Santos Customs Car Resale Prices
- GTA Online Jobs List & Guide
- Best Cars in Each Category
- List of Purchasable Cars
- List of Properties
- Gang Attack Locations Map
- Interiors & Enterable Locations
- GTA Online Rank Unlockables