According to Xbox spokesman Larry "Major Nelson" Hryb, World of Keflings will make its Xbox Live debut on November 1. While normally the game features a $10 price tag, anyone with an Xbox Live Gold subscription will be able to download it for free until the end of November. Iron Brigade joins World of Keflings on November 16, and will likewise be a free download for the remainder of that month.
As with all Games With Gold additions, once you've download either of these games, they're yours to keep - unless you delete them from your hard drive. If this occurs while the game is still a part of the Games With Gold promotion you can simple re-download the data, but if you wait until after the promo has expired your game is gone forever.
These two additions mark the second to last month of Microsoft's promotion. As the Games With Gold FAQ notes, "two free game downloads will be available monthly, starting on the 1st and the 16th of every month from July through December 2013."