To claim Obduction ® for free visit GOG.COM home page, scroll down and click the button on the giveaway banner to add the game to your library.
The giveaway ends on June 1, 1 PM UTC.
Its hidden amongst a bunch of other stuff so search carefully for the giveaway link: https://www.gog.com/game/obduction
A new sci-fi adventure from Cyan, the creators of Myst. Abducted far across the universe, you find yourself on a broken alien landscape with odd pieces of Earth. Explore, uncover, solve, and find a way to make it home.