To get the discount, click "Claim This Offer" on the page, then use your Reward Zone membership number when buying the game. You can also get this offer with an iPhone or Android by downloading the Best Buy Mobile App.
As per usual, you cannot redeem this discount on the website. In-store only.
Redeem it at this site, under "Gamers Club" > "Gamers Club Offers".
Edited by Forsvaine, 19 August 2012 - 10:13 AM.
GCU gets $30 off, not $20. Sorry for the confusion!