There are two ways that you can use to add games which are already installed on your system to GOG GALAXY.
- Using the 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 the 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 a long time ago. In these cases refer to the option below.
- Using the IMPORT FOLDER option allows you to add only one game per use. However, it will make GOG 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 the library click on LIBRARY on the top bar of GOG GALAXY). Then click on MORE -> Manage installation -> Import folder... , and select the game installation folder.