The "Add Credit to Multiple Cards" button is located in the "Loyalty Program" section of the Customers tab.
If you have a series of Gift Certificates that are in sequential order, you can use the "Add Value to Multiple Cards" Feature. To determine if they are in sequential order, ignore the first character (likely a "G") and ignore the last character. In the example below there are 6 sequential numbers from 2700-2705. Plug these values into the form as shown below. Be sure to check a few in the POS tab after you've completed this (enter *GiftCode* into the POS).
You don' t have card numbers yet you can create new ones.