Dress
Always wear dress clothes to an interview. Even minimum wage jobs you'll make a better impression in a pair of khakis and a button-up shirt. It may not be a great job, but there might be six other people applying for the job. You'd be surprised what a difference it can make.
Remove any nose, lip, etc. piercings. Ear piercings are fine, but the others should be removed. Remember, Gamestop sells a lot to kids and moms, so they want friendly looking people that won't scare their customers. Doesn't matter if you think they look fine, if anyone else wouldn't it could be a problem. Same with tattoos. If you can cover them with a long-sleeved shirt (or non-strappy shirt for women) then do so. Again, doesn't matter how you think they look, but a clean cut person with no tattoos and piercings will almost always get the job first.
Questions
Expect questions about your retail experience, attitude, game knowledge, and people skills. Be very positive.
Don't ever tell someone hiring that you don't get along with people. They do not want to hear that, even if it's true.
Don't complain about former bosses and jobs. They will assume you were the problem, not the other boss.
Don't come off as a know-it-all. Bosses hate these types of people for two reasons. One possibility is you are brilliant and you're going to make them look bad. The second, and more likely possibility, is that you're full of yourself and are going to be a pain to manage. Either way, nobody wants to work with you.