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 maximize it again, and not all window managers honor requests to unmaximize.

You can track the result of this operation via the state property, or by listening to notifications on the maximized property.