When attempting to redeem your GO Points gift card, if the attempt fails, you will normally receive a notification about why it hasn't worked. It is normally due to a typo when entering the card, or because the card has already been redeemed.
If it says your card has already been redeemed, you can check your overall GO Points balance when logged into your Gift Off account on the Redeem GO Points page.
If you are still stuck, then of course, please do contact support.