Rockstar calls it "the Los Santos spin on soccer": two teams take to the arena in souped-up cars and try to push enormous bombs into the opposition's goal. Make sure the ball isn't in your half when it explodes.
You can get the idea from the short clip below. It'll be available to play until January 14, and anyone that gets behind the wheel will receive double GTA$ & RP.
It's part of GTA Online's series of holiday updates, which will give players free gifts just for logging on. You'll get a gift every day between now and January 1—they're all listed here (scroll down to the "Festive Calendar" section), and they include sweaters, liveries and fireworks.
Additionally, you'll receive two free T-shirts if you play before January 7, which is also the cut-off point for double GTA$/RP for completing certain activities, including biker contract missions and gunrunning sales, and discounts on various items, such as hangars and aircraft. You can read about all them all in Rocktar's post.
Lastly, there's a new sleek car for purchase called the Grotti Itali GTO, pictured below. You can buy it from Legendary Motorsport.