Gtk.Window.prototype.set_modal

function set_modal(modal:Boolean):void {
    // Gjs wrapper for gtk_window_set_modal()
}

Sets a window modal or non-modal. Modal windows prevent interaction with other windows in the same application. To keep modal dialogs on top of main application windows, use Gtk.set_transient_for to make the dialog transient for the parent; most <link linkend="gtk-X11-arch">window managers</link> will then disallow lowering the dialog below the parent.