code: public ServiceFuture<List<ExpressRouteGatewayInner>> listAsync(final ServiceCallback<List<ExpressRouteGatewayInner>> serviceCallback) {
          return ServiceFuture.fromResponse(listWithServiceResponseAsync(), serviceCallback);
      }
docs: Lists ExpressRoute gateways under a given subscription.
  
  @param serviceCallback the async ServiceCallback to handle successful and failed responses.
  @return the {@link ServiceFuture} object