Something

void
gtk_expander_set_label (GtkExpander *expander,
                        const gchar *label)
{
  g_return_if_fail (GTK_IS_EXPANDER (expander));

  if (!label)
    {
      gtk_expander_set_label_widget (expander, NULL);
    }
  else
    {
      GtkWidget *child;

      child = gtk_label_new (label);
      gtk_label_set_use_underline (GTK_LABEL (child), expander->priv->use_underline);
      gtk_label_set_use_markup (GTK_LABEL (child), expander->priv->use_markup);
      gtk_widget_show (child);

      gtk_expander_set_label_widget (expander, child);
    }

  g_object_notify (G_OBJECT (expander), "label");
}