As you might expect, there is a catch – albeit a free catch. The first part is obvious: You'll need a Green Man Gaming account. You'll also have to sign up for Playfire, a service that tracks the games you play across multiple platforms (similar to Raptr). Finally, you'll have to link your GMG and Playfire accounts.
Once that's done, you'll be able to nab each game for free as they become available over the weekend.