There’s now an option to show a Discount Code field on the gift card form (Note: set to HIDDEN by Default).
When you enable it, a Discount Code field is displayed on the Gift Card Form.
1. Go to Settings > Promotions > Gift Cards
2. Check the “Show Discount Code” field on the gift card form
3. Click Save Changes
4. Go to Settings > Promotions > Discount Codes
5. Create a discount code for “Gift Card”
Now a Discount Code field is displayed on the Gift Card Form.