I preordered The Witcher 3 today and wanted to start the Preload via GOG Galaxy. On the first try i got an error about missing disc space, wich was caused by the default Install folder being on my quite stuffed C: drive.
After that i reassigned the Installfolder to another Drive ( 360gb left), wich somehow didnt fix the Error (it still says not enough Disc Space). Does anyone know if and how i can fix that? I somehow dont believe the game really needs more than 360 gb while downloading :D .

Sounds like it's just holding the info. Did you try restarting Galaxy? Don't just click the x, that will just minimize it.

You probably need to change the folder labeled "download games" as well.

I directed Galaxy to download to k: and install to c: as c: is my ssd but it's not very big. Wasn't enough space to both download and install as it has to extract files after it dowloads. This means you'll probably need somewhere around 50 gig free on 1 drive to both download and install there. I'm just estimating.

Quite interesting! Found another german user in a complete different forum with the same problem (guess it's not you LeCado?). He said, after installing the game for the third time, everything was fine.

Do the following:
1. Get offline installer.
2. Install the game.
3. Import the game in Galaxy.

It's likely Galaxy will pick the game up automatically, so you will need to follow steps 1 and 2 only.