Sometimes after GOG Galaxy's update, games can disappear from list of installed games.
Don't worry, all you need to do is to import your games back, no need to download them again.
Just open Galaxy and click GOG logo (upper left corner), click "scan and import folder" and select the path to your games.
In case if you want to import only one selected game please navigate to:
Library > Game > More > Manage Installation > Import folder
Select the folder and GOG Galaxy will import you game.