Ok, now you’ve cleared the hurdles. Yes, gift cards can be used to purchase an iPhone. If the balance on the card is high enough it will cover the cost of the iPhone and the sales tax. Tax will be charged for the state the iPhone is being shipped to. You cannot use Apple Gift Card to purchase Apple Care.
>>* In some countries and regions, your Apple Account balance can’t be used for some subscriptions like AppleCare+. Check if Apple Account balance in your country or region isn’t supported.<<
What you can buy with your Apple Gift Card or Apple Account balance - Apple Support
If you’re not purchasing Apple Card, it may not be a big deal. However, if you are, it may work in an Apple Store in US, but not online. You can call 1-800-MY-APPLE and confirm the use of your Apple Gift Card.
I hope you enjoy your new IPhone. 😀