1. Log into your GOG account. Navigate to your library and scroll to the bottom of the page. There, click on the "REDEEM CODE" link.
or you can go to this link directly: http://www.gog.com/redeem
3. On the next page, enter your unique code displayed on the ‘Orders’ page of your CDKeys account or on the link of your confirmation email if you had purchased as a guest, and click the green "Continue" button.
4. On the next page, you will confirm the order and make sure you are redeeming the correct game. Click the green "Continue" button to move to the final step.
5. On this page, you will "pay" for your order. As you have already purchased your key(s) from CDkeys.com this field will then show up as "Free". Don't worry, no more purchases are necessary. Click the green "PAY FOR YOUR ORDER NOW" button to finish your key redemption.
6. And that's it! From here, you may navigate to your GOG library where you will see the new title waiting for you.