I'll just say it was ... lower than 3.57. USC seems to be more focused on the type of person you are and if you bring the attributes they want while seeming able to keep up with the workload of medical school. (No, I don't know the attributes they want, sorry)
My MCAT was a couple points above their average, so that probably helped balance my GPA. I would say that my research and tons of leadership helped me, but then I also met a bunch of kids at my interview with very little interest in research, so I don't think it's something that USC necessarily wants to see in every applicant.
Feel free to PM me if you guys have more questions. Good luck and maybe I'll be seeing some of you once interviews start!