The GOG Summer Sale 2016 is officially live with new game deals and bundles every 24 hours.
You can get System Shock 2 for free (48 hours only) if you download the GOG Galaxy client (similar to Steam but for GOG games) and visit the sale page to claim the game, once the game has been claimed it will appear under your GOG games library.
Speaking of Steam, you can actually use GOG Connect to transfer some of your Steam games on to your GOG account.
Through out the sale you can collect XP to earn free games by participating in activies
You can participate in activities to collect XP and unlock free games, you'll be able to get:
Gabriel Knight: Sins of the Fathers – 20th Anniversary Edition
Dreamfall Chapters Season Pass