gtk:window-maximize (window)

window -- a gtk:window widget

Asks to maximize the window, so that it becomes full screen. Note that you should not assume the window is definitely maximized afterward, because other entities, for example, the user or window manager could unmaximize it again, and not all window managers support maximization.

It is permitted to call this function before showing a window, in which case the window will be maximized when it appears onscreen initially.

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