code: public Observable<ExpressRouteGatewayInner> getByResourceGroupAsync(String resourceGroupName, String expressRouteGatewayName) {
          return getByResourceGroupWithServiceResponseAsync(resourceGroupName, expressRouteGatewayName).map(new Func1<ServiceResponse<ExpressRouteGatewayInner>, ExpressRouteGatewayInner>() {
              @Override
              public ExpressRouteGatewayInner call(ServiceResponse<ExpressRouteGatewayInner> response) {
                  return response.body();
              }
          });
      }
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.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the ExpressRouteGatewayInner object