Here's a map showing all the GTA Online gang war / attack locations and times they are active.
There are around 56 gang attacks on the map and you can make around $2,000 per attack picking up crates (shoot them with a weapon) 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. Map updated to show 56 locations. Thanks to Whizwheels in the comments section for the update!)
Here is what the crates look like, they can contain $500, weapons, or ammo.
Thanks to tech for the map and Whizwheels for the rest of the locations.
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