cogadh: You have to do it one game at a time. Go into each game page in your library and click on the "More" button. One of those option will allow you to specify the games full install directory to look in.
almabrds: Can you post a image please? I didn't found this. I think when the client updated automatically, some files didn't download properly (I had to restart the program two times for the version change to 1.0.0.871).
cogadh: What game are you trying to get it to detect? Are you certain you specified the full install directory of the game, and not just the main GOG Games directory?
almabrds: I tried with the Hitman games.
Unfortunately, since all my games are already installed/detected, the menu options change, so I can't get an actual image or verify the text of the menu myself. However, the basic steps are this:
1. In your library, click on the game card for the game you want to detect. This should show you the game page with the big blue Install button and a More button next to it
2. Click on the More button, then Manage > Import Folder
3. Browse through the file tree to the game's full install directory, such as C:/GOG Games/Hitman and click OK
Galaxy should now detect that specific game. You will have to repeat the process for each individual game that Galaxy failed to detect.