I wouldn't go and say all Gamestops are the same. You just go to a bad store with a bad manager. I bet you right to corporate or call corporate office they will make it right. Complain above the store you go too.
I would be upset to buy a new game expect it to be new and find out it's been used. I didn't know Gamestop did that. Next time I go there and want a new game I will make sure & ask about that detail before I buy and make sure I can return it if I have problems with it.
I'm surprised they wouldn't refund your money or exchange the product for you.