The game uses Steamworks, so you’ll have to activate it on Steam. It has an 85/100 metascore.
You can open this page and it should have the countdown for when the deal goes live.
The deal is limited to the first 500 people. Around 1PM EST GameStop will release a coupon code that will give you the discount for the game. The best way to find out about the deal is to “Like” them on Facebook or follow them on Twitter. Or you can check their Flash Sale page. There’s a countdown on the page for your convenience.