code: def visit_GpxModel(self, gpx_model, *args, **kwargs):
          
          result = OrderedDict()
  
          put_scalar = lambda name, json_name=None: self.optional_attribute_scalar(result, gpx_model, name, json_name)
          put_list = lambda name, json_name=None: self.optional_attribute_list(result, gpx_model, name, json_name)
  
          put_scalar('creator')
          put_scalar('metadata')
          put_list('waypoints')
          put_list('routes')
          put_list('tracks')
          put_list('extensions')
  
          return result
docs: Render a GPXModel as a single JSON structure.