code: public Observable<ServiceResponse<P2SVpnGatewayInner>> getByResourceGroupWithServiceResponseAsync(String resourceGroupName, String gatewayName) {
          if (this.client.subscriptionId() == null) {
              throw new IllegalArgumentException("Parameter this.client.subscriptionId() is required and cannot be null.");
          }
          if (resourceGroupName == null) {
              throw new IllegalArgumentException("Parameter resourceGroupName is required and cannot be null.");
          }
          if (gatewayName == null) {
              throw new IllegalArgumentException("Parameter gatewayName is required and cannot be null.");
          }
          final String apiVersion = "2018-08-01";
          return service.getByResourceGroup(this.client.subscriptionId(), resourceGroupName, gatewayName, apiVersion, this.client.acceptLanguage(), this.client.userAgent())
              .flatMap(new Func1<Response<ResponseBody>, Observable<ServiceResponse<P2SVpnGatewayInner>>>() {
                  @Override
                  public Observable<ServiceResponse<P2SVpnGatewayInner>> call(Response<ResponseBody> response) {
                      try {
                          ServiceResponse<P2SVpnGatewayInner> clientResponse = getByResourceGroupDelegate(response);
                          return Observable.just(clientResponse);
                      } catch (Throwable t) {
                          return Observable.error(t);
                      }
                  }
              });
      }
docs: Retrieves the details of a virtual wan p2s vpn gateway.
  
  @param resourceGroupName The resource group name of the P2SVpnGateway.
  @param gatewayName The name of the gateway.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the P2SVpnGatewayInner object