A quick rundown of the modes, then: Raid sounds like standard CTF, with teams attacking bases to seize packages and bring them to their bases. Hold, meanwhile, is about finding packages and storing as many as you can in your base - unlike Raid, packages are also dotted across the map. GTA is like Raid, but with vehicles spread across the map. Finally, Contend is centered around a single vehicle that respawns after it's successfully captured.
According to Rockstar, the update is available now through Xbox Live and the PlayStation Network. It also adds a bunch of tweaks and fixes that you can check out here.