Even if they tell you over the phone that it cant be done, it may still work.
Most stores don't allow you to use a store gc to purchase 3rd party vendor gc's. It depends if its systematic or not. In Lowe's, the register won't let it go through. In staples, you make sure to get "a holiday cashier" and they don't know the rules and they do it for you.
(I used a staples cash card to purchase other gc's, I never tried a staples gc)
All I'm saying is that in some stores its systematic and in some its not, but most stores officially don't let.