GooglePay.js

$.when(adrma.fetchData({
        url: "https://pay.google.com/gp/p/js/pay.js",
        dataType: "script",
    }),

    adrma.fetchData({
        url: "https://js.braintreegateway.com/web/3.64.1/js/client.min.js",
        dataType: "script",
    }),

    adrma.fetchData({
        url: "https://js.braintreegateway.com/web/3.64.1/js/google-payment.min.js",
        dataType: "script",
})).then(function() {    

    // You will need a button element on your page styled according to Google's brand guidelines
    // https://developers.google.com/pay/api/web/guides/brand-guidelines

    var paymentsClient = new google.payments.api.PaymentsClient({
        environment: 'TEST' // Or 'PRODUCTION'
    });

    var gPaybutton = paymentsClient.createButton({
        onClick: () => console.log('TODO: click handler')
    });

    $("body").prepend(gPaybutton);

    var button = document.querySelector('.gpay-button');

    braintree.client.create({
        authorization: "sandbox_ykvt3vxx_hcrc97h9tfbz8b6c"
    }, function(clientErr, clientInstance) {
        braintree.googlePayment.create({
            client: clientInstance,
            googlePayVersion: 2,
            googleMerchantId: 'BCR2DN6T3PSN74SE' // Optional in sandbox; if set in sandbox, this value must be a valid production Google Merchant ID
        }, function(googlePaymentErr, googlePaymentInstance) {
            paymentsClient.isReadyToPay({
                // see https://developers.google.com/pay/api/web/reference/object#IsReadyToPayRequest
                apiVersion: 2,
                apiVersionMinor: 0,
                allowedPaymentMethods: googlePaymentInstance.createPaymentDataRequest().allowedPaymentMethods,
                existingPaymentMethodRequired: true // Optional
            }).then(function(response) {
                if (response.result) {
                    button.addEventListener('click', function(event) {
                        event.preventDefault();

                        var paymentDataRequest = googlePaymentInstance.createPaymentDataRequest({
                            transactionInfo: {
                                currencyCode: 'USD',
                                totalPriceStatus: 'FINAL',
                                totalPrice: '100.00' // Your amount
                            }
                        });

                        // We recommend collecting billing address information, at minimum
                        // billing postal code, and passing that billing postal code with all
                        // Google Pay card transactions as a best practice.
                        // See all available options at https://developers.google.com/pay/api/web/reference/object
                        var cardPaymentMethod = paymentDataRequest.allowedPaymentMethods[0];
                        cardPaymentMethod.parameters.billingAddressRequired = true;
                        cardPaymentMethod.parameters.billingAddressParameters = {
                            format: 'FULL',
                            phoneNumberRequired: true
                        };

                        paymentsClient.loadPaymentData(paymentDataRequest).then(function(paymentData) {
                            googlePaymentInstance.parseResponse(paymentData, function(err, result) {
                                if (err) {
                                    // Handle parsing error
                                }

                                // Send result.nonce to your server
                                // result.type may be either "AndroidPayCard" or "PayPalAccount", and
                                // paymentData will contain the billingAddress for card payments
                            });
                        }).catch(function(err) {
                            // Handle errors
                        });
                    });
                }
            }).catch(function(err) {
                // Handle errors
            });
        });

        // Set up other Braintree components
    });

});