So I had a evap code from the 08 cobalt I had and it uses the same gas cap as my 07 (no actual thread more of a bayonet lock). The seal on the cap is important as the car will pull a vacuum when running. The gas caps seal was not broken but it was heavily worn and lost all of its spongyness. I bought a replacement gas cap and the problem disappeared. I imagine the one on my GP will have the same fate but it is a cheap and easy fix.