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, e.g. the user or window manager, could unmaximize it again, and not all window managers support maximization. But normally the window will end up maximized. Just do not write code that crashes if not.

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 maximization via the "window-state-event" signal on the gtk-widget object, or by listening to notifications on the is-maximized property.