Your best approach is to create a file (say jscript_gsr_optin.php) in /includes/modules/pages/checkout_success. That newly-added file should look similar to

Code:
<?php
if (empty($order_summary)) {
    return;
}
?>
<!-- BEGIN GCR Opt-in Module Code -->

<script src="https://apis.google.com/js/platform.js?onload=renderOptIn"

async defer>

</script>

<script>

window.renderOptIn = function() {

window.gapi.load('surveyoptin', function() {

window.gapi.surveyoptin.render(

{

// REQUIRED

"merchant_id": "MERCHANT_ID",  //- Your Merchant ID value

"order_id": "<?php echo $order->info['order_id']; ?>",

"email": "<?php echo $order->customer['email_address']; ?>",

"delivery_country": "<?php echo (!empty($order->delivery['country']['iso_code_2']) ? $order->delivery['country']['iso_code_2'] : $order->billing['country']['iso_code_2']; ?>",

"estimated_delivery_date": "YYYY-MM-DD",  //- The purchase date/time is present in $order->info['date_purchased']