They're linked to your Samsung account.. Log in with the same one and they will all be there. You also get an email any time you get a gift card via Samsung Pay, which will contain the promo code to use it.
I figured this out. On the note 7 go to add gift card. On the old phone pull up your gift card in Samsung pay. Click view card number and use your finger print to show the whole card number. Input that card number into your note 7 and it should be there