Watch tire pressures especially as temperatures drop this time of year. For your GXP I'd consider GMPP tune. It'll give you more torque lower in the RPM band helping acceleration. The gasoline in the northeast is poor or mpg this time of the year. Along with temperatures I'm struggling to stay in the 30's.
Compared to my V8's the 4-cylinder is more sensitive to speed changes and weight, or even a head wind. Most of my mileage is on the highway, steady state driving yielding better mpg.
I'm REAL curious as to how you get your mpg numbers. I have an 07' gxp Sol w/auto trans and my normal mpg is around 20, with high 20's on highway. My average speed is 24mph.
My mpg numbers and the need to buy premium gas are my two biggest disappointments with my car.