I wouldn't do it.
If you're going to grad school you should be working on a paid assistantship as you learn a ton from that. I tell people not to go to grad school without one in most fields. Unless you're doing medicine, law, business etc. where they don't often offer them, then you live on loans and pay them off with the big bucks you make after graduating.
But you said "studying for grad school" so maybe you meant for entrance exams etc., in that case a part time job is ok. But you could still find something better than Gamestop who pays crappy and puts a lot of pressure on employees to get preorders, magazine subscriptions, warranties etc.