Q: How can I purchase iCloud storage as a gift for someone ?

The problem with this that it isn't a one-off payment. You have to give a credit card number, and then the monthly subscription is taken automatically until you cancel it. The credit card number would be the one in iTunes, so you couldn't use yours but then have theirs used to purchase songs or movies. Probably the only way would be for you to just give them the money, which isn't the nicest way of giving a gift.