Julie here from Shopify Support.
I'm glad to hear that you're testing out gift cards. As a merchant, it's important to understand how gift cards work prior to selling them.
When you issue a gift card, you generate a gift card code, and email it to a customer without receiving any payment in return. You decide how much the gift card is worth, and the customer who receives it can use its value toward other products in your store.
Gift cards are just like currency, so if you issue a free gift card worth $15, you're essentially giving customers $15 worth of merchandise for free. Simply put, since you didn't actually collect payment for the gift card, there is no payment to receive.
I hope this makes sense, but let me know if you require any further clarity!