Hello all:
I just wanted to post a possible solution if you're having problems installing a game and it tells you there's not enough disk space. This happened to me today:

I bought the Double Dragon trilogy and when trying to install it I got the not enough disk space on c error (e8). That wasn't the problem because I have ove 300 gigs of free space available and the game is 150 megs.

The solution I came up with was to delete the folder GOG Galaxy created for the game and try installing again and it installed with no problems.

Hope this helps.

Cheers!

Sounds more like a permissions error that it mistakes as running out of space.

This was what actually helped me with Witcher 3. Delete the Games folder inside the Gog folder and the newly created folder for the witcher.