Happenned to me just yesterday, I opened Galaxy and realized that KCD 2 disappeared from installed games... when checking on computer drive, all files were still present, but game executable was missing...

Anyway, when I went back to Galaxy, I choose to "Manage installation" and then "Import folder", where I chose the folder with game files on my drive... after this game update started running and it repaired the installation, so if anybody meets same problem, this is probably working solution...