I've just ordered Witcher 3 for PC from Amazon but I didn't realize that it comes on bluray (at least what the description says) and my PC does not have bluray reader. Will I be able to download it from gog.com or Steam using cd-key that came with the game?
Each retail PC copy of Wild Hunt will contain an additional GOG.com game code for The Witcher 3: Wild Hunt. One can install and play the game without the game code and without becoming a GOG.com user.
GOG.com game code grants access to a digital backup copy of The Witcher 3: Wild Hunt on GOG.com and to full software support, i.e. future updates, as well as all the additional content like the 16 Free DLCs, the FLAC & MP3 soundtrack, comics, wallpapers, art book, all voice over packs and more.
I asked CDProjekt (Witcher 3 support) directly and got following answer:
You will be able to download and install game, extensions, DLCs from GOG.com (using the GOG key included in box edition). Also optical drive is not (and will not) be needed to play. Game can be run from GOG games library. In addition the hardware requirements on the box does not contain any optical drive.