Mromson: I read on steam that Witcher 3 will be available on SteamOS, does that mean that pre-ordering Witcher 3 on gog.com will also grant me Steam keys, or is it better to just buy the game on Steam and then activate it on gog.com?
Well if the game is "really" available on SteamOS then it means that it will be available on Linux... and as GoG has said that they will start supporting Linux later this year it means that if you buy it on Gog you will most likely get the Linux version too (on top of the Windows and Mac one); version that you will be able to install on SteamOS should you want to.
Of course all of that is in the sole condition that they "actually" are going to release it on SteamOS and AFAIK there hasn't been any official confirmation yet.