As I can always remember being told, gift cards are like cash in the hands of others so guard them as you would your cash. Under common law a finder of lost property has a superior claim of right over any other person except the previous owner. Therefore the proper course is usually for someone finding an item to give the rightful owner an opportunity to recover the item after which it belongs to the finder. However in circumstances where it is impossible for the finder to find the true owner the law may allow them to keep the item they have found.
I am guessing that you don't know who the owner is anyway so its not like you can sue them for the balance of the card or anything.
