get_page_by_title() not returning anything [closed]

You are missing the second parameter in get_page_by_title. See get_page_by_title() reference. When you are testing without explicitly specified $output and $post_type, this function returns the post of type ‘page’ object by default. So you have to return $page->post_parent for patient’s parent page ID:

add_filter('gform_field_value_parent_id', 'parent_id');
function parent_id() {
    // three parameters here
    $page = get_page_by_title('TESTID', 'OBJECT', 'patient');
    // return patient's parent page ID
    if( !is_null($page) )
        return $page->post_parent;
}

P. S. It’s a good practice to enable WP_DEBUG in wp-config.php to see all errors and warnings during development stage. Often it helps to understand bottlenecks faster.