Typically the answer is NO but it's worth a try. Most retailers won't let you use a gift card or store balance on a previous purchase or to pay off an existing balance. It is usually to buy something else in the store. Again it's worth having a chat or calling the store to get an answer.
I understand. Yeah it's no big deal if they take it or not for the phone, but more saying what a smoking deal that would be for the 7 Pro, $545 off for the phone + $200 off with a gift card, that'd be $745 towards the new phone.
But they probably not doing that.