Copy this code into a snippet, then load that snippet into checkout.liquid

gp_google_autocomplete.liquid

<!-- Snippet gp_google_autocomplete -->
{% if gp_google_autocomplete %}
  {% assign gp_api_key = settings.gp_add_merchant_api_key | strip %}

  {% unless gp_api_key == '' %}
    <script>
      var pageChangeChecker = function(name, section, callback) {
        // checking for a specific class-attribute, else it's been erased by page:change
        if (!$(section).hasClass(name)) {
          if (typeof(callback) === 'function') {
            $(section).addClass(name);
            callback();
          } else {
            console.warn('pageChangeChecker requires a function for the callback parameter');
          }
        }
      };
    </script>
    <script src="{{ 'gp_checkout_autocomplete.js' | asset_url }}" defer="defer" async="async"></script>

    {% capture gp_api_url %}https://maps.googleapis.com/maps/api/js?key={{ gp_api_key }}&libraries=places&language={{ shop.locale | split: "-" | first }}&callback=setupGpCheckoutAutocomplete{% endcapture %}
    <script src="{{ gp_api_url }}" defer="defer" async="async"></script>
  {% else %}
    <script>
      console.error("No API key found for Google Places autocomplete");
    </script>
  {% endunless %}

  <script>
    var setupGpCheckoutAutocomplete = (function ($) {
      function setupGpCheckoutAutocomplete() {

        // Initialize Google Maps on for Shipping Address form and Billing Address form.
        if (Shopify.Checkout.step === 'contact_information' || Shopify.Checkout.step === 'payment_method') {
          var addressInput = $('form input[name="checkout[shipping_address][address1]"]').length ? 'shipping_address' : 'billing_address';

          function checkoutAutocompleteInit() {

            var placesAutocomplete = new GpCheckoutAutocomplete();

            placesAutocomplete.init({
              addressInput: addressInput,
              geolocate: {{ settings.gp_enable_geolocate | json }},
              countryError: {{ 'google_places.checkout.field_errors.country_invalid' | t | json }},
              address1ErrorPlaceholder: {{ 'shopify.checkout.contact.address1_placeholder' | t | json }}
            });
          }

          $(document).on('page:load page:change', function() {
            pageChangeChecker('autocomplete-active', '[name="checkout['+addressInput+'][address1]"]', checkoutAutocompleteInit);
          });
        }

      }

      return setupGpCheckoutAutocomplete;

    })(jQuery);
  </script>

{% endif %}