Is anyone able to install this? I've rebooted the computer, restarted the Galaxy client, a bunch of times, successfully installed other games, but this one called Trüberbrook that I just bought -- no. It completely refuses to even download. It says the servers are unreachable. But downloading anything else works just fine. Help?

Any luck if you go to the Extras tab and download the offline installer? If it successfully downloads, install the game by running the installer. Afterwards, click the cogwheel in the top left, and choose Add Games & Friends, then Scan Folders for GOG Games to add it to GOG Galaxy.

You can look up the various download error codes (E#) in the support center:
https://support.gog.com/hc/en-us/categories/201553025?product=gog