If you haven't already, set up an inventory item with Item Number like "GIFTCARDLOST". Make sure the item is not "stocking" and is not "taxable". Set it to Stop on price.
Check the balance check on the lost gift card. You will need the full card number to do this, see support if you need help accessing the full card number.
Enter a single transaction or if your system is configured to not allow combined sale/return transactions enter two separate transactions...
•Enter a return for item "GIFTCARDLOST".
•Issue the new gift card.