Gift Certificates is a "special" feature of Zen Cart ... the key to that is that they are marked as Virtual Products *and* their Model must start with:
GIFT<anything or nothing here>

This triggers various events that must take place on Gift Certificates for functionality of tracking, redeeming, mailing, etc. ...

Gift Vouchers/Certificates are "Cash" and must be treated separately from other Products so that they can work properly with the checkout or sending to a friend etc.