Try overriding the Gravity Forms default confirmation with a Popup Maker popup using either a GF hook or with jQuery.

alternative-jquery-solution.js

/* Try this code in the footer area of your page if the PHP hook below doesn't work. */
jQuery(document).ready(function ($) {

  	// Replace 123 and 456 with your respective popup IDs throughout.
	$("#pum-123").on("pumBeforeOpen", function () {
		const $gfConfirmation = $("#gform_confirmation_message_2"); // Replace 2 with your form's ID.
    		if ($gfConfirmation.length) {
			console.log(`%c[PUM] Confirmation loaded for Gravity Forms form #2.`, 'color:orangered;font-size:16px'); 
			PUM.preventOpen(123); // First popup with the GF form.
			PUM.open(456); // Thank you popup.
		}
	}); // Listener

}); // jQuery

gravity_forms_confirmation_override_for_popup_maker.php

<?php // Ignore this first line when copying to your child theme's functions.php file.

/** 
 * || Gravity Forms Confirmation Override 
 *
 * Reference https://docs.gravityforms.com/gform_confirmation/
 */
function custom_confirmation_message( $confirmation, $form, $entry, $ajax ) {
    $confirmation = "<script>window.top.jQuery( document ).on( 'gform_confirmation_loaded', function () { PUM.close( 123 ); PUM.open( 456 ); } );</script>";
    return $confirmation;
}
add_filter( 'gform_confirmation_2', 'custom_confirmation_message', 10, 4 );

// Replace 123 and 456 with your popup IDs and add the updated snippet to your child theme's functions.php file.