Apple policy on gift cards states that they can change at any time what products you can buy with an Apple gift card. They recently did not allow gift cards to buy ipads.
Go to apple online store search for a gift card and read the fine print at the bottom.
An Apple gift card is not a 100% guarantee you will be able to get the product you want at a certain time.
The reason they did that with the iPad they say is because people were using gift cards to get around other policies meant to prevent resale.
No telling if they will do it for the iPhone or if they ever have, but be advised that is in fact their policy on gift cards.