I wrote:
Now if I try to launch gsmartcontrol from the Gnome Shell, I get
the prompt from userhelper for my password, but it fails to run.
From the command line, the same thing happens, but at least it
shows the error message "Gtk-WARNING **: cannot open display: :0".
I withdraw the question. After rebooting the system, gsmartcontrol and
gparted both work fine. I'm not sure what went wrong before.
Eric