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

window -- a gtk:window widget
monitor -- a gdk:monitor object to go fullscreen on

Asks to place the window in the fullscreen state on the given monitor. Note that you should not assume the window is definitely full screen afterward, or that the windowing system allows fullscreen windows on any given monitor.

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