how can I redeem a gift card in iCloud storage

iCloud storage is charged to your general iTunes account. Redeem the gift card to that.

Redeem App Store & iTunes Gift Cards, Apple Music Gift Cards, and content codes - https://support.apple.com/HT201209

Gift cards are redeemed onto your Apple ID, and then their balance can be used to pay for items from the iTunes, App and iBooks stores as well as iCloud storage, and Apple Music. Gift card balance is then automatically used when payment is made for any of those charges.

See here for how to redeem a gift card onto your account: Redeem App Store & iTunes Gift Cards and content codes - Apple Support