Eu configurei manualmente, mas é irritante.
widget = self.builder.get_object('textbuffer_text')
#~ self.settings.bind("text", widget, "text", Gio.SettingsBindFlags.DEFAULT)
text = self.settings.get_string("text")
widget.set_text(text, len(text))
...
def on_textbuffer_text_changed (self, widget):
b,e = widget.get_bounds()
self.settings.set_string("text", widget.get_text(b,e,False).decode('utf-8'))