The only references I can find on the internet indicate that the command should be
But that results only in the error message:
$ gnome-session-save --logout-dialog
From reading the skimpy man page for gnome-session-save, this would seem to make sense; the command ends a session of gnome, not a session in the gnome display manager.
** (gnome-session-save:3594): WARNING **: Failed to call logout: The name org.gnome.SessionManager was not provided by any .service files
So, this still leaves me with no way to log out. I can reboot or shut down, but that's it.