I would start with the gas cap as well. And when replacing it, make sure that your filler neck is in good condition. One car I had, the gas cap was brand new. But the filler neck was rusted and not sealing well, giving me this code. Replaced the filler neck with a good one from the yard, and all was well.

But like scotty said, could be a vacuum leak as well. But I would start with the gas cap.