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.

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