posting here bec i dono where else to:
can i buy amazon gc and use it to get year of prime in order to avoid the sales tax?
If you buy a gc, it just goes toward the total price of your purchase. It doesn't change the taxes on the purchase.
If there was a 'gift of prime' gift card, then you would pay applicable taxes upon purchase (maybe in a no sales tax state) and then no taxes when you apply it to your account to get prime (maybe in a sales tax state).