If you're in stop and go conditions a lot, you'll never get decent mileage. If you drive an automatic -- same deal.
However, if you're driving an M6 and are getting that kind of mileage -- you either have a heavy foot, a hole in your gas tank, or your wife is siphoning gas out of your car to top hers off.
The owner's manual contains the ideal shift points for the GTO -- from an economy standpoint. For the '04, they are:
First Gear: 0 - 15
Second: 16 - 22
Sixth: 51 and up
Real fun, huh? Those shift points are something like 1500 RPM -- so that will give you an idea of just how limiting they are. You've 5.7 to 6.0 litres of space to fill -- so it's going to consume a lot of gas if you get any RPMs going at all.
Beyond the shift points, if you have a manual, you can do stuff like coast downhill or coast up to stoplights, etc. But nothing raises your overall MPG like a nice freeway trip. In my '04, I'm able to get around 17 MPG city -- but that's using every trick in the book.
Also, depending on where you live -- the formulation of "winter gas" may be such that your mileage gets taken down a couple of points.