GtkBuilder accepts UI descriptions and turns them into Gtk applications. In this case, the UI description is in the file and can be read by the get_object() method.

Load the UI file using GtkBuilder and get the window object using the Name (or ID) chosen in Glade, and add the window object to the application:

    _onStartup() {
        let builder = new Gtk.Builder();
        this._window = builder.get_object('window1');