//1. replace line 39 with
add_filter('gform_get_form_filter', array($this, 'output_message') );

//2. And add a new function inside the class:
function output_message() {
        $GWpage = get_page_by_title( 'The Page Title' );
        $GWpost = get_post($GWpage->ID);
        $GWcontent = $GWpost->post_content;
        return apply_filters('the_content', $GWcontent);
    }