gtk:window-unfullscreen (window)

window -- a gtk:window widget

Asks to toggle off the fullscreen state for the window. Note that you should not assume the window is definitely not full screen afterward, because other entities, for example, the user or window manager, could fullscreen it again, and not all window managers honor requests to unfullscreen windows. But normally the window will end up restored to its normal state. Just do not write code that crashes if not.

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