Give it awhile. Sometimes it can take a few days for your free gift to be processed through. However, don't worry, it's yours. If you still haven't received it after a predetermined amount of time, just contact support. They will check it out and add the game to your library (as long as you claimed it within the giveaway duration).

Remember to check your spam folders as such, in case the email notifying you about your free game goes there.

Just to be sure: does the giveaway banner on the front page look like the attached image now?