When you get a person who wants to BUY a Gift Certificate, sell them one in the House Accounts Screen (Front Desk | House Accounts).
Make sure you place the Gift Certificate number in the Description field so that you have a note attached in BookingCenter that shows which Gift Certificate number you sold, to whom, and on what date.
Here is an Example. For step-by-step instructions, see each section.
Gift Certificate Sold: A Gift Certificate is sold for $150 to Kim Johnson. It expires 1 year from purchase date.
Redeeming the Gift Certificate: Then, 5 months later, a guest presents this Gift Certificate as Payment.