gtk:window-fullscreen (window)

window -- a gtk:window widget

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

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