Beware you also need to account for sales tax in almost all states as regular gift cards are taxed in every state with sales tax.
So I'm not 100% sure on this fact. It could vary depending on the gift card purchased or it could perhaps vary by state, but in my experience, any total requested on a gift card hasn't been subject to sales tax.
For instance, a few weeks ago, I took advantage of the Family Dollar deal on PSN cards and paid $127.50 for $150 of PSN credit. Given that the promotion was 15% off cards that week, I didn't have any sort of sales tax added (based on the math/total). Likewise, with the 10% off gift card promotion today at Target, no sales tax was added to my gift card purchases. Again, not sure if it varies from state to state, but my impression has been that in most instances sales tax isn't assessed until one makes a purchase with the gift card (tax assessed on the item purchased, versus the currency).
Regarding using the hypothetical savings from this promotion for a future next gen console purchase, people earlier in the thread have correctly pointed out that a $500 GameStop gift card wouldn't cover the full cost of the console, due to the tax being added to the $399.99/$499.99 console price after the fact; however, there would still be the roughly $60-75 savings one would get from the discount on the gift card from this deal (since one would currently pay $340 for a $400 card and/or $425 for a $500 card). And if one were so inclined, you could indeed estimate the additional tax cost based on your county/state residency and also buy a second, lesser card that would save 15% off the tax as well.
If you are getting taxed on the gift card at the time of purchase, that would SERIOUSLY suck as it would mean you're effectively being taxed twice. I would be interested in hearing whether other CAGs are paying sales tax for gift cards, and if so, what state you're living in. For reference, I'm out of NC.
And for those still considering this deal, you have just a few more hours to capitalize!