Gift cards used on iCloud?
If I use an iTunes gift card on my iTunes app, will the credit also be used to pay for my iCloud?
iPhone 6s Plus, iOS 10.1
Yes.
iCloud storage plans and pricing - Apple Support
Accepted payment methods for iCloud storage upgrades include credit cards, debit cards, and iTunes Store credit. If you don't have enough iTunes store credit to complete your upgrade, you'll be charged the remaining amount. Apple Store gift cards aren't accepted as payment for upgrading iCloud storage. Learn how to manage the amount of storage you're using.