I implemented Guake's code to Gtk3 by myself.
first connect motion notify event, resizer is paned object.
self.resizer.connect('motion-notify-event', self.on_resize)
next write resize event
def on_resize(self, widget, event):
if Gdk.ModifierType.BUTTON1_MASK & event.get_state() != 0:
mouse_y = event.device.get_position()[2]
new_height = mouse_y - self.get_position()[1]
if new_height > 0:
self.resize(self.get_allocation().width, new_height)
self.show()
now you can expand your window with gutter.