They don't publish the avg. stats unfortunately (I frantically looked when I was considering the school myself). Despite that, I know that the avg. GPA is definitely NOT lower because its a newer school. The truth is that SPPD is a very very old school -- only the name is new. The school has been around for 81 years...
SPPD Facts - USC School of Policy, Planning, and Development
If you have those many units and that GPA, you probably will not get an SGR, unless you're still taking a pre-req. Even then, I was taking 2 pre-reqs in the spring, and only had 44 units by the end of fall, and I still got a straight acceptance.
As far as advice goes, all that I can offer is get the best grades that you are capable of, make sure you have finished the pre-reqs (macro+microecon, finite math or pre-calculus), and have started working on classes that articulate to satisfy the USC general education curriculum. What do your extracurriculars look like?