gtk:window-fullscreen-on-monitor (window screen monitor)

window -- a gtk:window widget
screen -- a gdk:screen object to draw on
monitor -- an integer which monitor to go fullscreen on

Asks to place the window in the fullscreen state. Note that you should not assume the window is definitely full screen afterward.

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