code: func (gcm clientMethod) ReturnTypes() string {
  	var types []string
  	for _, resp := range gcm.SuccessRespBodyTypes() {
  		types = append(types, resp.Type())
  	}
  	types = append(types, []string{"*http.Response", "error"}...)
  
  	return fmt.Sprintf("(%v)", strings.Join(types, ","))
  }
docs: // ReturnTypes returns all types returned by this method