Also, they're letting you connect games you own on Steam to your GOG account now:
https://www.gog.com/connectThough just a few days ago the page showed way more stuff for me like Saints Row 2, Braid, FTL, Trine, Unreal, Unreal Tournament, etc. than it does now. So "limited-time" is way more limited than I thought.

EDIT:
UPDATE: Last chance to grab your games from the original list! Make sure to get them until June 8, 12:59 PM UTC.
Today, we're launching a new program called GOG Connect. The premise is simple: connect your Steam account and add your eligible games to your GOG.com library.
Whether you're checking us out for the first time or have been with us for a while, GOG Connect gets you DRM-free versions of your games, digital extras, and a whole lot of freedom of choice (like whether you go with the GOG Galaxy client or not). It gets you our take on game ownership, and we say: why buy the games more than once?
Thanks to our awesome partners including Deep Silver, Harebrained Schemes, Jonathan Blow's Number None, TaleWorlds and more, you can now add more than 20 games to your GOG.com library if you previously purchased them on Steam.
The full list of games will always be available on connect.gog.com, starting with these and more:
- The Witness
- FTL: Faster Than Light
- The Incredible Adventures of Van Helsing: Final Cut
- Galactic Civilizations 3
- Trine Enchanted Edition
- Saints Row 2
- Shadowrun Returns
- The Witcher: Enhanced Edition
While GOG Connect will stick around, the available games will come and go. These are limited-time offers made possible by participating developers and publishers, so stay tuned as we bring new titles onboard in the future (and grab your copies before they go away)!
Also of note, Saints Row 2 on GOG apparently has some extra fixes that the Steam version doesn't. (Appropriate since a defunct arm of CD Projekt did the shit port.) Though I cannot personally confirm this and are going off the GOG thread and what some people on GAF said.
Even though it's not up on the page anymore, any Witcher title's CD Key activates on GOG too.