How can I manual make coupon code as giftcard?

You can write the predefined coupon codes in CSV with a specific format. After that, you can import the CSV under the “Import/Export” tab. After importing the coupon codes when any gift card product will get purchased then imported coupons will get assigned to the Gift cards. You can Import your own coupon codes along with all required fields for that which you may check from provided CSV file format, and after that whenever any customer purchases the Gift card he/she gets the Imported Coupon Code rather than the one generated by the system.