MrBMT: Make sure you're logged in, then click here to refresh your games library:
https://www.gog.com/user/refresh Close Galaxy, then re-open it - it should no longer say "Not yet available" or show you the "Connect now" button.
Note: It may take half an hour or so after clicking that link before it works correctly via Galaxy or it might just be there was previously an issue with Galaxy when I first tried it earlier today.
Either way, I can confirm (at least a little while after refreshing your library) it now lets you install the game via Galaxy, therefore if you import a folder where you've already installed the game I assume it should update it for you.
alburl: Don't really understand what's going on. It said that Dreamfall Chapters was unavailable in Galaxy, so I followed your instructions and it became available. I have Book 2 already installed and completed. So I tried to import folder and it said "Not Authorized". So then I tried to install and it said "Checking local files". It took a few seconds to do this and then started installing. Only thing is it is installing 9.8GB. Why is it doing this if the local content is there, apart from Book 3? Surely Book 3 isn't that big?
I backed up the game's folder beforehand, just in case, as I've heard reports of games getting completely overwritten with Galaxy.
Any ideas what's going on? I'll leave it installing and see if it's wiped my saves.
Regarding gogalaxy waiting about an hour, after you say that you can install it.
You can safely delete the second book, in fact, the third update of the book is not added ... basically if you download the book 3, you find yourself also book 1 and 2.
For saved unless you delete them should not be a problem.