Some of my games, while not being listed in the installed section, nevertheless cannot be imported by the normal "import folder" (option is simply not there). These games do have a list of executables, but the verify option is greyed out. Only the Install button is available. What's up with that?

Have you tried Cogwheel - Add games and friends - Scan folders for GOG games?

I only have the Import Folder option for games that are owned but not installed. I don't know how you get games in this sort of halfway state, where they're not read as installed but you have executables linked to them.

If the problems persist, you can try starting over with a fresh database:
1. Make sure GOG Galaxy isn't running.
2. In the File Explorer address bar, go to C:\ProgramData\GOG.com\Galaxy\storage\
3. Move the galaxy-2.0.db file to the desktop.
4. After restarting GOG Galaxy, import your installed games as mentioned above.

If the games aren't very big, you could also just press that Install button. Verify/Repair would probably have the same effect anyway, if GOG Galaxy decides the whole thing needs to be reinstalled.

This worked for me. I had to mark the game "uninstalled" after moving it to my new drive and THEN I was able to import it.