As reported downstream by a xfce user that wouldn't like to have to install nautilus for this:
http://bugs.gentoo.org/show_bug.cgi?id=350919

Looks like g-d-u is hardcoded to nautilus instead of trying to use alternatives. This is probably related with they following I found in src/palimpsest/gdu-section-volumes.c:


static void
on_fs_mount_point_element_activated (GduDetailsElement *element,
                                     const gchar       *uri,
                                     gpointer           user_data)
{
        GduSectionVolumes *section = GDU_SECTION_VOLUMES (user_data);
        GError *error;
        gchar *s;

        /* We want to use nautilus instead of gtk_show_uri() because
         * the latter doesn't handle automatically mounting the mount
         * - maybe gtk_show_uri() should do that though...
         */

        s = g_strdup_printf ("nautilus \"%s\"", uri);

        error = NULL;
        if (!g_spawn_command_line_async (s, &error)) {
                GtkWidget *dialog;
                dialog = gdu_error_dialog_new (GTK_WINDOW
(gdu_shell_get_toplevel (gdu_section_get_shell (GDU_SECTION (section)))),
                                               gdu_section_get_presentable
(GDU_SECTION (section)),
                                               _("Error spawning nautilus:
%s"),
                                               error);
                gtk_widget_show_all (dialog);
                gtk_window_present (GTK_WINDOW (dialog));
                gtk_dialog_run (GTK_DIALOG (dialog));
                gtk_widget_destroy (dialog);
                g_error_free (error);
        }

        g_free (s);
}

But maybe this situation can be improved :-/

Thanks a lot