Any gadget out there that works like the g-tech but not only does it show hp etc, but also shows fuel mileage and miles till empty, etc

 

No. Not yet at least

That stuff is pretty LAME for sport's car's anyway's. My buddy has all that crap on his ZO6 and he say's it's pretty inacurate. Too many different driving patterns on sport's car's. How can it accurately calculate your MPG when you might decide to do some whooping up on a cobra or something on the way to the grocery store

 

The "MPG computers" need to be connected to (or built into) the PCM. They use the injector pulse widths to measure fuel consumption, integrating those values over time. Accuracy is limited, but not all that bad in many factory setups.

Not aware of anything that can be added on to the 4th Gen F-Body, but I've never really looked, since you can get a pretty good idea of your mileage and miles to empty by resetting the trip odometer at fill-up, and watching the fuel gauge.