In order to transfer ETH (or the gas currency for your network), it is recommended to use TranferEth.CreateTransaction 
EthTransaction transferTransaction = await TransferEth.CreateTransaction(client, wallet, recipientAddress, DecimalNormalizer.Normalize(1));
TransactionReceipt receipt = await wallet.SendTransactionAndWaitForReceipt(client, transferTransaction);
Behind the scenes, this will create a GasLimitEstimator which will construct the transaction for you - including determining the appropriate gasPrice, gasLimit, and nonce.