Payment for iCloud storage
Can you still use iTunes gift card to May monthly iCloud storage charges
iPhone 6, iOS 12
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Yes.
Redeem App Store & iTunes Gift Cards and content codes - Apple Support
Or you can use the credit to upgrade your iCloud storage or pay for subscriptions like Apple Music, Apple Arcade, Apple TV+, Apple TV channels, or Apple News
But I am pretty sure you will also need a billing method other than Gift Card to start it. And you will not be able to remove that without first stopping the subscription.
But as long as you have credit, it will charge the itunes store credit first.