There are no tutorials, but there is no reason why you couldn't create a gift certificate implementation.
You could sell gift cirtificates as products and add field to your order details table that is the gift cirtificate ID which you should create using a random value so it can be used as the "coupon code".
Then on your cart page add a recordset that will look up the coupon code from the gift certificate items in the order details database to return the amount remaining.
You would probably have to write custom code to subtract the correct amount used from the original coupon purchase on the confirm page once something is paid for, but it shouldn't be hard to have the amount used in a session variable as well as the coupon code to identify the record to update and to update the record and subtract the amount used with simple math.
