You can't download a giftcard to a Magicband, they do not hold any type of cash or credit value. You can charge things back to your Disney resort with a MB, but it goes on your hotel folio, not automatically deducted from a gift card. If you have a plastic gift card, you can transfer the balance to it, and you can probably save it to an Apple wallet if you have an Apple device. You could also print out the gift card and take it with you.