If you have a gift card that also has a separate PIN, you'll still be able to use it (as long as it hasn't expired!).
Just copy and paste the code and PIN together in the ‘Discount code or gift card’ box at checkout. Your original expiry date and all the terms and conditions for the voucher will stay the same.
For example, if the details were:
Gift Card Number: 12345678
PIN: 5555
You’d enter ‘123456785555’ and hit ‘Apply’.