Only Promotions created for Services and Series are available for sale through the Cash Register. You will not see Gift Certificate Promotions available for selection there.
Promotions created for Gift Certificates can only be purchased online by Clients through the Promotion's designated webpage.
If a Client wants to purchase a Gift Certificate Promotion in person, sell the Gift Certificate like normal and apply a discount on the first page of checkout. The Client will pay the discounted rate, but the gift code will still be good for the original amount of the Service or Custom Certificate.