Ne0plex: SOLUTION:
Download the "GOG Downloader"
Go to this link: www.gog.com/ downloader/ listing/ the_witcher_3_wild_hunt (without the spaces)
Click on the files that you want to download.
The files should now show up in the downloader.
Moriah21: start from 0% ............
True, but only once. It seems with the "user-friendly" Galaxy Install approach, you start the download from the scratch each time there is an error during the download/install.
At least by downloading the installers (and manually installing them, and then importing the installed game to Galaxy, in case you want to play it through Galaxy and not independently), you don't have to try to download dozens of gigabytes repeatedly over and over again. In case you prefer not to have to do that. If the game needs a re-installation, just run the TW3 installer you already have on your hard drive.
Sometimes the older way is the more convenient way, as you are more in control then...