Can I forward a gift card I received to someone else?

Yes, or you have have it sent again.

Click on this link to find out how to 'Resend iTunes Gifts' --> Gift apps, music, movies, and more from Apple - Apple Support

- "If you sent an iTunes Gift and your recipient can't find the link to redeem it, you can resend them the gift from your iPhone, iPad, or iPod touch or your computer. When you resend an iTunes Gift, the recipient will receive another email with a link to redeem the gift."

How did you resolve this?

Your granddaughter is 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.

Apparently you can, if you purchase it in the wrong storefront. I live in Germany, but managed somehow to purchase a gift card that can only be redeemed in the U.S. Storefront. 🤦🤦🤦🤦🤦🤦