Does the GTO have a 6-speed tranny? I can't remember, but I'd imagine that'd be of some assistance when trying to hypermile.
Oh, and if it ever comes down to it, you've got plenty of rear-end gears to choose from, as all those drag-racers are dropping their taller ratios like hot potatoes
The good thing about starting off with "terrible" mileage is that any changes you make will pay you back
MUCH faster than if you started with a higher MPG car.
If a Hummer driver gains 1mpg, they save ~$500/year.
I'd have to gain about 15mpg to get the same savings....