We are truely sorry this has happened. =(

This issue has been fixed and the game should reappear in your GALAXY library soon. If you don't wish to wait you can either reinstall GOG GALAXY or follow these steps:
- Close GALAXY
- Remove the following folder: C:\ProgramData\GOG.com\Galaxy\storage
- Launch GALAXY again

However this will remove all your plugins and you will have to reconnect all of your integrations as well.