There's a Collector's Edition of the game on Amazon and I was wondering whether this comes with any way to redeem the game on GOG or whether you could redeem it like the Witcher (getting it on Steam and putting in the CD-Key into the proper GOG form)? Or is the Steam version client-free and only needs it to download the game and has no need for it when starting the game (e.g. Grand Theft Auto: San Andreas)?

The retail version of the game comes only with a steam key, so it is not DRM free.