For what it's worth, I'm having the exact same problems as the OP. Preloaded many days ago in Galaxy, have been patiently waiting... and Galaxy still thinks I need to wait.

So I've taken to downloading all of the installer parts via the website (and the patch, and the free DLC, and...) and we'll hope the "brute force" method works.

One last thing that comes to mind, did you select to install games inside galaxy folder? It's a known issue if you do that.

If he's getting the same error I got, it's that the manual patch doesn't detect that the Witcher 3 is even installed (when GoG Galaxy did the partial-pre-release-install).

Probably because the Galaxy install just stages files but doesn't write anything to the Registry until the "final part" is downloaded and executed... and Galaxy doesn't think the game is released yet!

[
update: Actually, it's because the patcher looks for the .bin files, but Galaxy extracts them. See here:
https://www.gog.com/forum/the_witcher_3_wild_hunt/is_anyone_in_from_gog_galaxy_yet/post8
]

Good thought, but not in my case. I have Galaxy configured to install Games under:

F:\Games\GoG\

(And I can confirm that all of the Witcher 3 files are staged there under a folder:
"F:\Games\GoG\The Witcher 3 Wild Hunt"

While my Galaxy client lives in:

D:\Games\GalaxyClient

In a few other threads it was hinted that issues with the manual patch and galaxy install often come down to moving the game after it was initially installed. There was also a manual fix posted for that, but I'll have to look for it.

Probably it would be best to post a issue report at galaxy bug tracker: http://mantis.gog.com

I know nothing about Galaxy, have never used it or even read a tutorial for it, so this is just me talking out of my own ass, but is there any chance that Galaxy downloaded the install files and kept them somewhere? It probably puts them in a /temp or something but if the files are still there you may be able to just manually pull the 25MB installer and do a full manual install?

Downloading everything from the Account page here worked just fine for me, but I can imagine you wouldn't want to re-download 20+ gigs if you didn't have to.

No, I never moved the game.
When I got GoG Galaxy I changed the settings of where it installs games, downloads, etc. Because the initial folder locations were really dumb.
So I made folders inside GoG.com called Games and Downloads, one for installing games and the other for downloads, respectively.
I have installed 3 other games with GoG Galaxy just fine and they run just fine and have no issue. But the Witcher 3 does not seem to want to start at all.

I have a feeling it might be because the patcher error logs seem to refer to another folder where my Witcher 3 game isn't. located, because that's not where I have my game installs located on GoG Galaxy.
But if that really is the case, then it's really dumb to have the option to even allow people to install to different locations if their patcher only updates to a specific folder location that god knows how many people changed when installing their games.

I am completely out of options, so I am uninstalling the entire thing and installing it again via GoG Galaxy.
Please someone here who can actually run Witcher 3, post a screenshot of your folder setting locations on GoG Galaxy. I am betting that is the source of all this hassle. Which does nothing but infuriate me if that really is the issue because why would they even allow to change folders if they're not going to accommodate an update that works for the folder you want the game to be in.

I checked, but Galaxy deleted the files automatically from the temp download folder after extracting them to the install location. Efficient, but in this case, deleterious. (hehe)
Actually, you've still got the option to download the [many installer files] directly from the website and skip Galaxy altogether. That's your safest bet. It's what I'm doing...

(and FWIW, I did the same as you.. I changed all the defaults because they were poor defaults for my system)

...you receive no points for that pun, sir, and may God have mercy on your soul.

(If your internet speed is terribad or metered, you can probably recover the files as long as you don't have a secure delete program running on your computer. If you have decent, uncapped internet, it's probably easier and faster to just re-download)

You're right. That is the safest bet. GoG Galaxy isn't doing any favours.

*grin* *tips hat*

Yeah... my internet is decent. Almost done with the re-download... At least I had AdVenture Capitalist, my 3 year old misbehaver, and this here forum to keep me distracted in the meanwhile!

Happy to report that I got all the files downloaded and I'm installing now.

I checked the "Verify Integrity Before Installing" option just to be anal-retentive about it. I'm not taking chances.

EDIT: AND IT WORKED! I'm able to launch the game now. Hooray.

Good find. I annotated to bug with my $0.02

Goto https://www.gog.com/account
Sign in, click your Witcher 3 game, and wallah~ Choose the file you wished to download.

So Witcher Pre-load thingy that is not "DRM" actually bites.

Hey, I know it's quite a bit late, but I was having the same problem. Thanks for posting this, that's what I'm going to try. GOG Galaxy seemed so promising.