gtk:window-unmaximize (window)

window -- a gtk:window widget

Asks to unmaximize the window. Note that you should not assume the window is definitely unmaximized afterward, because other entities, for example, the user or window manager, could maximize it again, and not all window managers honor requests to unmaximize. But normally the window will end up unmaximized. Just do not write code that crashes if not.

You can track maximization via the "window-state-event" signal on the gtk:widget object.