Add body class if title is hidden in GeneratePress (https://generatepress.com/)

add_body_class_if_title_hidden.php

<?php
/**
 * Add body class if title is hidden.
 *
 * @param  array $classes An array of body class names.
 * @return array          An array of body class names with added class name if title is hidden.
 */
function add_body_class_if_title_hidden( $classes ) {
	if ( ! generate_show_title() ) {
		$classes[] = 'gp-title-hidden';
	}
	return $classes;
}
add_filter( 'body_class', 'add_body_class_if_title_hidden' );