Pros
Although you don't actually get to play video games, you do get to talk about them, and you get to be exposed to games that you might not have noticed or be interested in otherwise. Dealing with people isn't that bad, and it's always good to get retail experience. There's also something pretty satisfying about being able to genuinely help somebody, even while you know the company is ripping them off every time they want to trade in a used game. At first, working at GameStop isn't so bad - in fact, it's kind of fun. However, as time goes by you'll want to leave more and more; I'd recommend staying for no longer then a month.
Cons
Whenever the store is in a lull, employees have to be constantly moving and getting the store in "optimal condition." This means that every single game is alphabetized, the floor is completely clean and vacuumed, and every single drawer is in perfect condition. If you're going to work at GameStop, I'd highly recommend coming in as a seasonal employee during the December holidays. That way, at least you'll get to interact with customers instead of having to interact with empty game cases. Another horrible side to working at GameStop is having to push reservations and magazine subscriptions to people who clearly don't want it. As an employee, your only value to the company is the number of preorders and subscriptions you get, and that will be made very clear to you.