The listing, Gigantic [bonus key]Ezren Ghal Hero has ended.
http://gigantic.wikia.com/wiki/Ezren_Ghal/Hero
You do not redeem these keys within Steam, you must redeem them from the Gigantic client using the instructions below.
To Redeem Your Gigantic Prize:
Visit www.GoGigantic.com/steam
Click on the “Play Game” button and download Gigantic.
Launch the game and create an Arc account.
Once in the Gigantic launcher, select the “Redeem Code” button.
Enter the code and click “Next” to claim your pack.