Google Pay on your own checkout page: decryption is done by us

// If you want to use Google Pay in your application, you are required to register
// with Google in order to receive a Google Merchant ID
var paymentProductSpecificInputs = {
  googlePay: {
    merchantId: '<Your Google Merchant ID>',
    merchantName: '<Your merchant name>',
    gatewayMerchantId: '<Your merchant ID with us>'
  }
};
// Retrieve the Google Pay Payment Product via the client to server API
// It is assumed that the session, paymentDetails and paymentProductSpecificInputs objects here are all valid
session.getPaymentProduct(320, paymentDetails, paymentProductSpecificInputs).then(
    function(paymentProduct) {
    
        const allowedCardNetworks = paymentProduct.paymentProduct320SpecificData.networks;
        const tokenizationSpecification = {
            type: 'PAYMENT_GATEWAY',
            parameters: {
                gateway: paymentProduct.paymentProduct320SpecificData.gateway,
                gatewayMerchantId: paymentProductSpecificInputs.googlePay.gatewayMerchantId
            }
        };
    
        // Store the payment product in the payment request which is required for
        // creating the encryptedCustomerData later on
        session.getPaymentRequest().setPaymentProduct(paymentProduct);
    },
    function() {
        // Google Pay is not available, offer your customer another payment option to complete the payment
    }    
);

// If you want to use Google Pay in your application, you are required to register
// with Google in order to receive a Google Merchant ID
var paymentProductSpecificInputs = {
   googlePay: {
    merchantId: '<Your Google Merchant ID>',
    merchantName: '<Your merchant name>',
    gatewayMerchantId: '<Your gateway ID with us>'
   }
};
//https://developers.google.com/pay/api/web/reference/request-objects#TransactionInfo
var transactionInfo = {
    currencyCode: '<Currency code>',
    totalPriceStatus: '<FINAL>',
    totalPrice: '<Amount>'
};

//Retrieve the Google Pay Payment Product via the client to server API
// It is assumed that the session, paymentDetails and paymentProductSpecificInputs objects here are all valid
session.getPaymentProduct(320, paymentDetails, paymentProductSpecificInputs).then(
    function(paymentProduct) {
    
        const acquirerCountry = paymentProduct.acquirerCountry;
        if (acquirerCountry != null) {
            transactionInfo.countryCode = acquirerCountry;
        }
    },
    function() {
        // Google Pay is not available, offer your customer another payment option to complete the payment
    }
);

function addGooglePayButton() {
    const paymentsClient = getGooglePaymentsClient();
    const button =
        paymentsClient.createButton({
            onClick: onGooglePaymentButtonClicked
        });
    document.getElementById('container').appendChild(button);
}

function onGooglePaymentButtonClicked() {
    const paymentDataRequest = getGooglePaymentDataRequest();
    paymentDataRequest.transactionInfo = getGoogleTransactionInfo();
    
    const paymentsClient = getGooglePaymentsClient();
    paymentsClient.loadPaymentData(paymentDataRequest).then(function(paymentData) {
        // handle the response
        processPayment(paymentData);
    }).catch(function(err) {
        // show error in developer console for debugging
        console.error(err);
    });
}

function processPayment(paymentData) {
    var paymentToken = paymentData.paymentMethodData.tokenizationData.token;
    @todo process the paymentToken as described in the next step
}

{
    "mobilePaymentMethodSpecificInput": {
        "encryptedPaymentData": "{\"signature\":\"MEUCIQCcSASjWu3ax9jQ8qX/+rsNLZ95uiyxamXiKnHlJi8A1QIgZj ...",
        "paymentProductId": "320"
    }
}

// This example assumes that the SDK is initialized correctly
var paymentRequest = session.getPaymentRequest();
paymentRequest.setValue("encryptedPaymentData", "{\"signature\":\"MEUCIQCcSASjWu3ax9jQ8qX/+rsNLZ95uiyxamXiKnHlJi8A1QIgZj ...")

var encryptor = session.getEncryptor();

encryptor.encrypt(paymentRequest).then(function(encryptedString) {
    // Send the encryptedString to your backend. This exact String is the expected value for the
    // encryptedCustomerInput in the server to server Create Payment API
}, function(errors) {
    // Inform the consumer that something went wrong
});

{
    "order" : {
        "amountOfMoney" : {
            "currencyCode" : "EUR",
            "amount" : 2300
        },
        "customer" : {
           "merchantCustomerId": "<Your ID of the consumer>",
           "billingAddress" : { 
                "countryCode" : "NL"
            }
        }
    },
    "mobilePaymentMethodSpecificInput" : { 
        "paymentProductId" : 320, 
        "encryptedPaymentData": "{\"signature\":\"MEUCIQCcSASjWu3ax9jQ8qX/+rsNLZ95uiyxamXiKnHlJi8A1QIgZj ..."
    } 
}

{
    "order" : {
        "amountOfMoney" : {
            "currencyCode" : "EUR",
            "amount" : 2300
        },
        "customer" : {
           "merchantCustomerId": "<Your ID of the consumer>",
           "billingAddress" : { 
                "countryCode" : "NL"
            }
        }
    },
    "encryptedCustomerInput" : "<Encrypted string>" 
}