There are two ways that you can use to add games, that are already installed on your system, to Galaxy.
- Using SCAN AND IMPORT FOLDERS option allows you to add multiple games in one swoop.
To scan for games which you have already installed, click the GOG.com icon in the upper-left corner of GOG Galaxy main window, and select SCAN AND IMPORT FOLDERS from the drop-down list. Select your base GOG Games folder (the one that stores multiple game installation folders), and see what appears on your games list.
This method might not work with games installed long time ago. In such case refer to the point below.
- Using IMPORT FOLDER option allows you to add only one game per use. However, it will make Galaxy attempt to use the existing files no matter what, even if the game couldn't be found automatically.
To use this option, find the game in your library (to open library click on LIBRARY on the top bar of Galaxy). Then click on MORE -> Manage installation -> Import folder... , and select the game installation folder.