Why would you want to? Not only will the conversion be expensive but you won't be saving any money.
E85 gas mileage is your mileage with regular gas -5 to -7 MPG. So if you would be getting 27 MPG with gas, you'd be getting 20MPG with E85. I looked on the fuel economy for the Impala flex fuel to figure that out so I'm assuming it will be the same for the G6 (it doesn't have the G6 flex fuel vehicle in its stats on fueleconomy.gov.)
E85 is cheaper than regular gas. However, you'll be filling up more often because you get alot less MPG and any cost savings will be erased. The stats on the site say it's actually a few dollars more than if you just filled it up with regular gas.
So in my opinion, you will just be wasting your money.