Announced earlier yesterday, Anvil Gate will act as a bridge between the events Gears of War 2 and Gears of War 3 (there's a slight delay between the two). As such, it will deal with an inevitable event: a Lambent advance across the planet. But rather than detailing a full-scale between the COG and the new foes, Anvil focuses more on a specific instance--the defense of Jacinto. Here's the synopsis:
With the Locust Horde seemingly destroyed, Jacinto's survivors have begun to rebuild human society on their island stronghold. Raiding pirate gangs take a toll--but it's nothing that Marcus Fenix and the Gears can't handle. Then the terrifying life-forms they thought they'd left behind--the Lambent, creatures even the Locust feared--begin to advance across the planet. Gears and gangs must fight side by side to stop their deadliest enemy yet, falling back on the savage tactics of another bloody siege: Anvil Gate.
Sci-fi writer Karen Traviss is penning Anvil Gate. You might know her. She's been responsible for a bunch of Star Wars expanded fiction novels, the Wess'har Wars series, and even a Halo book. Traviss is also penning Gears of War 3 proper's story, some related comics, and has written other Gears of War books, so she has an obvious investment in Anvil Gate and Epic Games' property as a whole. That's a good thing.
That said, I doubt there's a need to read anything to understand what will be going on in the Gears of War 3. Chainsaws, cursing, muscles, and aliens that need killing are all pretty easy things wrap your head around.