I think more than anything it is all about fit. This is my first time applying, so I cannot say from personal experience, but I think the POI will just get a sense whether or not you willl fit well into the lab. Of course this includes many factors like past research experience, independent research work (honors thesis/fellowships), expressed interest in the POI, the lab, and the school, how you "mesh" with the POI and the grad students, and questions you may pose regarding future research directions. I think once you get to the interview point, your GPA and GRE scores won't matter. They chose you for a reason. They already see you as a potential member of their lab.