code: public ServiceFuture<ExpressRouteGatewayInner> getByResourceGroupAsync(String resourceGroupName, String expressRouteGatewayName, final ServiceCallback<ExpressRouteGatewayInner> serviceCallback) {
          return ServiceFuture.fromResponse(getByResourceGroupWithServiceResponseAsync(resourceGroupName, expressRouteGatewayName), serviceCallback);
      }
docs: Fetches the details of a ExpressRoute gateway in a resource group.
  
  @param resourceGroupName The name of the resource group.
  @param expressRouteGatewayName The name of the ExpressRoute gateway.
  @param serviceCallback the async ServiceCallback to handle successful and failed responses.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the {@link ServiceFuture} object