The game will be available for free starting today through April 8.
It was announced today during CD Projekt's Spring Conference as a way of saying thank you to existing customers.
GOG is undergoing changes, with the site beginning to offer newer titles in addition to PC classics.
Starting today users can also purchase both Assassin's Creed: Director's Cut and Heroes of Might & Magic V as a bundle for $19.99 through April 12. After April 12 the two titles will be sold separately for $19.99 each.
Users can also now pre-order Botanicula through GOG, which will be available April 19.
Last year, GOG reached 6 million downloads for the service's third anniversary.
Watch the intro to Fallout below: