When Gift Certificate is sold, the sale is recorded in the House Accounts. See Gift Certificates | Sell
When the Gift Certificate is redeemed, it is presented as a payment method by the guest at the time of check out. This means that it is applied to the Guest Folio as any other payment would be applied. In this case, using the Receipt Type "Gift Certificate Redeemed" or whatever you choose to call it. See Gift Certificates | Setup
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.