To redeem a key on GOG, you simply need to log in with your GOG account or create a new one. Then go to the GOG Gift Redeem page or if the link is not working go to the bottom of the GOG page and click “Redeem Gift”. Simply input the key in the text field and press continue (as shown on the image bellow). Remember to mark your key as used afterwards.
Note: GOG.com codes expire on a set date, which can be found in the appropriate Groupees offer description.