I have a couple questions that should help narrow down the issue.
Are you getting a specific error code? If so, what is it.
Have you tried 'purchasing' it through Xbox.com?
Some of the conditions for the Games for Gold is: "
Each participating Xbox 360 game (selected by Xbox) available for free download from Marketplace only during specified half-month window; available games may vary by country. Some titles may only be available for download by adult account holders 18+. Kinect
and/or hard drive required for some games. Offer available in all Xbox Live countries where Xbox Live Gold memberships are supported while supplies last, is non-transferable, not redeemable for cash, and cannot be combined with any other rebate or offer."
Are you in a Live supported country?
Let us know so we can help figure this out.
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