If you find this error in the confirmation page of any Gravity form then follow these simple steps to resolve it.
function WP_List_Table::get_columns() must be over-ridden in a sub-class.

Add this code in form_settings.php and notification.php under the class GFConfirmationTable extends WP_List_Table {

function get_columns() {
    $columns = array(
            'name' => 'Name',
            'subject' => 'Subject'
            );
    return $columns;
}

…and WoooHoo! it works magically!! 🙂