I would account for it as a contribution equal to the value of the card, assuming that the issuer is still in business and in a position to redeem it.  It would increase your net assets in an equal amount.
On the Form 990 tax information return I would treat it as a noncash contribution.  Although the instructions are not entirely clear on the point, it seems less like a check, which is essentially the equivalent of cash and would be treated as a cash contribution, and more like a bond or note redeemable by the issuer, which would be treated like a security and as a noncash gift.   Unless it is a very large amount, however, I don’t think I would lose much sleep worrying over the classification.