Our Galaxy Team is investigating the issue. The following workarounds should help.
1) Aborting the installation and import already existing files.
To do this you will have to abort the installation (clicking MORE > Manage installation > Verify / Repair and aborting it might be required) and use the MORE > Manage installation > Import folder option to get the Galaxy Client to recognize the files on your drive.
2) Installing to a different directory.
Simply selecting to install one more time, but to a non-default directory might work.
3) Using the standalone installers to install the game.
To download them manually click MORE > Backup & goodies and select the Game Backup Copy.
You can click the magnifying glass icon next to the entry on the download list to the left to open the download directory, it is stored in the "Other Downloads folder".
To change this directory, open the Galaxy Client, click the GOG icon in the top left corner, select Settings and change it 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.