The next free Xbox game you'll receive as an Xbox Gold subscriber is Magic: The Gathering - Duels of the Planewalkers 2013, according to a snap of Xbox Australia's home page picked up by Reddit user TheGesio.
Update - Microsoft has confirmed that Magic: The Gathering 2013 is September's first free Xbox Gold game, followed by Rainbow Six: Vegas on the 16th. More here.
Original story - Take a look at the tile in bottom right, and compare it to the screenshot further down this page. Assuming TheGesio hasn't fabricated the whole thing for shizzles and/or gizzles, it's pretty conclusive.
Magic: The Gathering probably isn't the blockbuster free game some Gold subscribers are waiting for, but it is, nonetheless, a rather excellent game. You may wish to check out Log's review of the previous instalment while I push Microsoft for comment. More shortly.