I have now implemented

    function getindex(builder::GtkBuilder, widgetId::String)
        convert(GtkWidgetI, ccall((:gtk_builder_get_object ,libgtk), Ptr{GObjectI},
            (Ptr{GObjectI}, Ptr{Uint8}), builder, bytestring(widgetId)))
    end

which allows to

    using Gtk
    b = GtkBuilder("C:\\test.glade");
    showall( b["window1"] )