I realize this thread is closed but doesn't the problem lie with gedit more than gksu. I get absolutely no warnings or errors substituting leafpad for gedit.
Code:
cavsfan@cavsfan-MS-7529:~$ gksu leafpad /etc/fstab
cavsfan@cavsfan-MS-7529:~$
Code:
cavsfan@cavsfan-MS-7529:~$ gksu gedit /etc/fstab
(gedit:6506): Gtk-WARNING **: Calling Inhibit failed: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files
cavsfan@cavsfan-MS-7529:~$
Both times I made a change and saved the file.