Gst::QueryDuration::create() returns a Glib::RefPtr<Gst::Query> object instead of Glib::RefPtr<Gst::QueryDuration> which requires a dynamic cast for each call to the methods of Gst::QueryDuration, like this:

Glib::RefPtr<Gst::Query> query = Gst::QueryDuration::create(GST_FORMAT_TIME);

dynamic casting:
gint64 duration = Glib::RefPtr<Gst::QueryDuration>::cast_dynamic(query)->parse();

the "right" approach: return a Gst::QueryDuration object and just do this:
gint64 duration = query->parse();

So, please change the create() constructor of Gst::QueryDuration so that it returns a Gst::QueryDuration object (as expected) instead of Gst::Query.