Open popup after Gravity form submission - More on https://timersys.com/how-to-display-a-popup-after-user-submits-form/

functions.php

<?php // don't add this tag

// Print javascript code in the footer of the site to trigger the popup upon form submission
add_action('wp_footer', 'add_popups_trigger_code',150 );
function add_popups_trigger_code(){
    // show code on certain page, comment out to print code on every page
    if( ! is_page(520) )
        return;
?>
<script>
    jQuery(document).ready(function(){
 
        jQuery(document).bind('gform_confirmation_loaded', function(event, formId){
            if(formId == 1) {
                // run code specific to form ID 1 and popup 123
                SPU.show('529');

            } else if(formId == 2) {
                // run code specific to form ID 2
                SPU.show('529');
            }
        });
 
    })
</script>
<?php
}