Gtk.Window.prototype.set_decorated

function set_decorated(setting:Boolean):void {
    // Gjs wrapper for gtk_window_set_decorated()
}

By default, windows are decorated with a title bar, resize controls, etc. Some <link linkend="gtk-X11-arch">window managers</link> allow GTK+ to disable these decorations, creating a borderless window. If you set the decorated property to false using this function, GTK+ will do its best to convince the window manager not to decorate the window. Depending on the system, this function may not have any effect when called on a window that is already visible, so you should call it before calling Gtk.Widget.prototype.show.

On Windows, this function always works, since there's no window manager policy involved.