Just FYI, you have to be a GMG newsletter subscriber or it won't work. From the email fine print:
*Voucher valid until [5th June 2015 and 5pm UTC].
Voucher only valid on Batman: Arkham Knight or Batman: Arkham Knight Premium Edition.
May not be used in conjunction with other vouchers or GMG credit.
User must be subscribed to newsletter to redeem voucher.