I had the same problem. No files were available for download, not even the old ones. Clicking the refresh link in this thread helped and now everything works and i see the download files.

I have been logged in the whole time so maybe thats why it won't work if you don't refresh. Actually steam has the very same issue, there you have to log out and back in for new episodes in episodic games to work. Me and many others have had this same problem on steam too with episodic games, many people have been confused there too every time a new episode is coming. Maybe logging out and in in gog would help too instead of the refresh link.

I would recommend if you have problems first try to log in again and/or use the refresh link posted in this thread. Hope it helps!

Linux release confirmed - thanks! (mac has a new filename as well)

Not directly related, but thanks for that link. This helped me to get the Chapters page to display game files in the first place (the game file section was completely empty for me so far).

None of what you've mentioned fixed the Connect button for me, any other ideas or ideas as to why it didn't help?
I refreshed via that link, shut GOG off (even went as far as restarting my PC) and logged back in again. When I click on the game under my installed games it shows in the changelog that "Patch 3.0.0.3 Adds Book 3 to the game" but the connect button is still there.
P.S. I've downloaded zhe game manually before Galaxy was able to download it from my library and haven't moved any files/folders and Galaxy now lets me run it.

It takes about an hour after refreshing your account before you can use it via Galaxy. I think you need to quit & re-open Galaxy after an hour before it updates properly too.

Can't download it at all. Not through GOG Galaxy, and not through the site.

Heck, I can't even install it on galaxy :(

You're welcome! :) Glad I could help!

Thank you for your answer :)
I ended up uninstalling Chapters and uninstalling then re-installed Galaxy, which got me into a mess as Galaxy didn't uninstall the game properly so I kept getting "not authorized" when I tried to download the game again so I had to find the remaining files manually and delete them, after which it all worked fine.
Talk about the longest journey to get it working properly ;D
Have you tried doing it recently?
Until a couple of days ago I used to have the same problem with not being able to download it via Galaxy and, until a couple of minutes before my post, I couldn't download it manually either as none of the game files were showing up in my library.
To say the least, it freaked me out xD

I followed an advice I've found here, posted by the same person who now tried to help me out, which said:
MrBMT: "Make sure you're logged in, then click here to refresh your games library: https://www.gog.com/user/refresh "
This solved my missing files problem.

You might try uninstalling and then re-installing Galaxy. You won't lose any of the games you've installed and after you install it again it'll automatically connect with the games again and they'll show up for you to play and then you could try installing Chapters.
This solved several problems for me

So, if I've understood correctly, I have to redownload the whole game from GOG.com? And it won't overwrite my save from the end of Book 2? I've managed to get the GOG downloader to start downloading "Dreamfall Chapters Season Pass" (5.55 GB). Is that it?

Excellent!! That solved my problem

Thanks!!