American Express Gift Card magically has a $0 balance.?

Using a prepaid/gift card at a gas station is always a bad idea. Most gas stations put a hold of $75 on any debit card until the transaction posts. This is a pre-authorization charge. This can take up to 7 business days. A credit card will only get charged for the amount owed but debits don't have that restriction & gift cards are considered debit cards for transactional purposes. The money will be returned but you will have to wait and, next time, just pay cash for gas, it is a lot simpler.