According to a screenshot of the Australian Xbox 360 store, Magic: The Gathering 2013 will be the Xbox Live Gold free title for September.

You can see the game on the bottom right of the dash, in a tile with the “Games with Gold” text on it.

magic xbox live gold 600x336 Rumor: 'Magic: The Gathering 2013' is the Xbox Live Gold free title for September | VGLeaks 2.0

Of course, remember that this info is a rumor and has not been confirmed by Microsoft.

Thanks, VG247.