GOG Connect is the ability to own games on GOG, that you already have on Steam. Alongside the currently sale going on at GOG, the team have added in new titles for you to bring over to GOG for free.
All you have to do is have an account at GOG, and own the games on Steam already. Then you will be able to grab them on GOG as well. It's a great thing for GOG to do, as gamers are quite accustomed to buying in one place and having access to their games in many places.
They didn't give us an exact list of what's included, but they said:
QuoteMetro 2033 Redux, HuniePop, Broken Sword Director's Cut & Broken Sword 2 Remastered, Starbound, The Witcher 2 and way more.
There's a total of 20, we know that much at least.
The new games will be available to grab until November 13, 10:59 AM UTC.