I usually don't post about huge sales anymore, since I figure most people who are interested are probably already aware of them, but this time GOG is doing something like what you might see Steam do for a Summer sale.
First of all, and totally unlike Steam, you can get a free copy of System Shock 2
for simply visiting their site using the GOG.com Galaxy Client.
Secondly--and this is the part more like Steam--you can earn XP and badges from completing certain tasks. But again unlike Steam, this will earn you actual free games instead of just stupid wallpapers or emoticons like it would on Steam. The first one you can get, meaning the one you will most likely be able to qualify for, is Spelunky
Check this page to see your progress and ways to earn XP for free (as opposed to earning XP from buying games during the sale):https://www.gog.com/summer_xp