you need to add the following lines to english.php

DEFINE('BOX_HEADING_GV_ADMIN', 'Gift Voucher');
DEFINE('BOX_GV_ADMIN_QUEUE', 'Gift Voucher Queue');
DEFINE('BOX_GV_ADMIN_MAIL', 'Mail Gift Voucher');
DEFINE('BOX_GV_ADMIN_SENT', 'Gift Vouchers sent');