Since you sent the Gift Card from Apple's website and not from the App Store, the previous link is not going to be much help and the steps to resend the Gift Card will not work. There is one important note that may apply:
- The recipient of your gift must have an Apple ID in your country or region. For example, if you're in the United States, you can't send a gift to a friend who lives in Australia.
I suggest you contact Apple directly for any resolution as sending the Gift Card on Apple's website should only take 1 hour. Let us know what Apple has recommended and the solution that was provided.
Contact Apple for support and service - Apple Support