On Windows this issue can occur if the installation of a redistributable package (like: Visual C++, NVIDIA PhysX, DirectX) finishes with en exit code that is not recognized by GOG GALAXY. Our GOG GALAXY Team is investigating and working on a solution.
As a workaround please try using the offline backup installers.
To download them manually click MORE > Backup & goodies and select the Game Backup Copy.
After the download finishes, click the arrow down in the bottom left corner and the magnifying glass icon next to the game's title. This will open the download directory.
To change this directory click the GOG GALAXY icon in the top left corner, select Settings and change the "Other Downloads folder" in the Downloads section.
After downloading the standalone installer you need to right-click on the executable and select "Run as administrator" to start the installation process.
If you wish to help us investigate the issue you can send us log files.
Please close the GOG GALAXY Client and go to his directory:
You can access the directory using the Run... window (WinKey+R key combination) and entering:
Add the contents of the above directory to a zip or rar archive and attach them to your support request. Click here to open a new request. Please include the title of the affected game.