I've backed the game on Kickstarter and will be getting the physical copy and I'll activate it on Steam. But if we get the chance to activate it on GOG instead I'll much rather do that.

So if anyone know the answer on this you're free to answer. But I'll also update you with info if I get an answer from the devs or any other backer.

Update:
We did get to choose where we wanted to activate the game, on Steam or GOG, so I have ofcourse choosen GOG.
And just for the info. The cd-key that comes with the CD is for Steam only, it does not work here on GOG. GOG sais it's not one of their keys.

Now we'll just wait and see if we get the artbook and stuff on GOG as on Steam.

I didn't get the option to choose though, or the email must have surpassed me. I would love to have the game on GOG though. Let's hope it will get added to Steam Connect ^.^