Gamestop will not suffer from this in the slightest.
They will happily print "secondary use fee" activation codes for you right from the register, just like they already do for online passes, and take their cut from it.
For the thousandth time - despite all the noise, none of these companies really have it out for Gamestop. The Gamestop corporation is a business partner which sells new consoles, accessories, and games for them.
You know who doesn't sell new product for them? You. If it weren't for you trading in or selling your games, the secondary market wouldn't exist. If it weren't for you loaning out your games to all your goodtime buddies, everyone would have to buy their own copies. You're the reason they don't get their cut, so all these plans are, and always have been, about getting that cut from you. Never forget that. Gamestop is going to roll right along, and it will cost them nothing. It's only going to cost you.