GDL_DOCK_OBJECT_UNSET_FLAGS (ph, GDL_DOCK_AUTOMATIC);

should be moved into _init()

 if (object) {
        gdl_dock_placeholder_attach (ph, object);
        if (position == GDL_DOCK_NONE)
            position = GDL_DOCK_CENTER;
        g_object_set (G_OBJECT (ph), "next-placement", position, NULL);
        if (GDL_IS_DOCK (object)) {
            /* the top placement will be consumed by the toplevel
               dock, so add a dummy placement */
            g_object_set (G_OBJECT (ph), "next-placement", GDL_DOCK_CENTER, NULL);
        }
        /* try a recursion */
        do_excursion (ph);
    }

Should be converted to construct-only property.

Feel free to create a patch otherwise I will try to fix it when I have time.

This is fixed in master now - sorry that it took so long! I hope people will use your gdlmm bindings!