gift card to buy iphone

This article explains the different kinds of gift cards Apple uses and has links on how to redeem them and what can be purchased with each kind. Click on the link --> What type of gift card do I have? - Apple Support

What you can buy with your Apple ID balance - Apple Support

This is a long document but if you read it all it will have useful information for you in this ssituation:

Payment, Financing, Refunds and VAT - Shopping Help - Apple