I have no real knowledge of the XML serializer. We register all other namespaces in advance, therefore I would expect that also the new GSAK namespace needs be registered. See

   gpx.attribute(PREFIX_XSI, "schemaLocation",
                PREFIX_GPX + " http://www.topografix.com/GPX/1/0/gpx.xsd " +
                        PREFIX_GROUNDSPEAK + " http://www.groundspeak.com/cache/1/0/1/cache.xsd");