FAQ on Gift Certificates:
https://www.zen-cart.com/tutorials/index.php?article=64
When you set up a gift certificate and the customer buys it (completes payment and checkout), they are sent an email with a redemption code. When they redeem that code either by clicking on the link or by keying in the code on the checkout-payment page (or the gv-redemption screen), then the customer will have GV funds available to them for use in that or future transactions.
The customer can then use it during a transaction of their own, or they can forward parts or all of it to others by supplying an email address. If they email it to someone, the redemption process starts again with a new redemption code ...