Q: Can I use an Itunes gift card to purchase more ICloud storage?

Welcome to the Apple community.

Credit/Debit card only I'm afraid.

I added an iTunes gift card to my account earlier this year, and my iCloud storage auto-renewal was automatically paid for using the credit.

Yes, you can use iTunes Store credit to pay for iCloud storage just fine.

iCloud storage 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.

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 storag

I think you may be overlooking the OP's circumstances here. You can use credit to pay for storage on an ongoing basis, but you cannot take out more storage unless you have a credit card registered to your account.