The no tax thing is only w/Gamestop trade-in cards (which are red with that bunny on them) or trade credit put on a Powerup Rewards card (black or silver) not the standard gift cards. Also not all states are tax-free if you pay fully in store credit. Check the GS trade-in thread to see if your state is tax-free if paid fully in credit.
Yeah my state is. But I also have one of those bunny cards that I traded something in, and I've been adding credit to it.