If you use GeForce NOW and a standard gaming PC, using a non-first Display Adapter, users will likely see a black screen on your first launch in GeForce NOW, because GeForce NOW still does not have your savegame info yet since it has not synced from Steam or Epic.
If you encounter a black screen at start, simply restart your game and watch for a cloud sync warning that should only appear once. Make sure you select the saves you wish to keep, whether its from the Steam or Epic cloud (which syncs from your gaming PC) or the local saves which are from GeForce NOW. You may need to do this several times if you will keep playing God of War on gaming PCs with a non-first display adapter. This happens because the information about the game’s progress is stored with the graphics settings in a single file.