So you have downloaded the addon with your browser and you weren't able to install it?
Is your game up-to-date (version 1.10)?
If yes: Is it shown as 1.10 in GOG Galaxy?
If not do this:
GOG Galaxy is not recognizing patch 1.10 (even though it is installed)
There is a possible workaround for this:
1) Shut down GOG Galaxy,
2) Download and install the offline patch 1.10 (from
https://www.gog.com/account/ or Galaxy's MORE -> GET BACKUP & EXTRAS - if you used Galaxy to download, make sure to close it before installation!),
3) Delete this folder: C:\ProgramData\GOG.com\Galaxy\storage\1207664643\
4) Launch GOG Galaxy,
5) Wait until it verifies the game files (this will take a while),
6) Galaxy will download some changed/missing files, if needed.
This should ensure that Galaxy displays the correct game version.
Or do this:
Deinstall Galaxy and install it again. Import Witcher 3 to Galaxy and hope it will recognize this time that Witcher 3 is version 1.10 now.
== IMPORT ALREADY INSTALLED GAMES TO GALAXY ==
1. For GOG games installed using installers from the last few months (so called Galaxy-compatible installers)
Click the "+" button on top of the sidebar and select "scan and import folders" - it will find all compatible games within that folder and add them to the Client.
2. For remaining GOG game INSTALLATIONS
Find the game in the Library (click on the image of the game), then click the More button and select "Manage" -> "Import folder" and point the folder selector into the folder where that game is installed.
Please be aware that Client will accept any folder pointed to and will overwrite files inside it (this can for example wipe some mods or damage other apps in case of pointing to a wrong folder) and will update the game to the newest version (current default language English).
Source:
http://www.gog.com/forum/general/gog_galaxy_beta_2/post1 This work for someone before:
http://www.gog.com/forum/the_witcher_3_wild_hunt/gog_galaxy_patch_version_not_match_with_the_game_version/post2 After that install the addon again.
