In order to claim your free copy just head on over to GOG.com's Galaxy page. There you can sign up to join the beta test of Galaxy and recieve your free key for the game in the next few days.
A little history lesson here for those of you who haven't played the game: Alien versus Predator Classic 2000 was originally released back in 1999 and was developed by Rebellion. This version is the re-release that was made available in 2010, updated to make it work on modern PCs. The game covers three campaigns spanning the perspectives of the Colonial Marines, Alien, and Predator. Despite it being free right now I'd be recommending it you to pick it up anyway, as the title says, it's a classic.