What's the best gas to use for my Grand Prix GTP

Most importantly Non-Ethanol and min 91 octane

Tune your engine and fuel system to run E85

if your not looking to tune it. go with Shell or BP premium. You will pay a little xtra compaired to your local stations but for about the past 10 years they have gone back and forth for the best fuel for your dollar.

Agreed with Shell Hi Octane, not so much with BP. Reason being some of the BP stations are not turning over their gas fast enough this creates situation with gas that is less than "Fresh". Would not recommend changing over to E85. You will have somewhat better performance at the sacrifice of fuel system and engine components. Again this depends on the mods you have done to your GTP. Most people don't know that it takes 1.5 gallons of gasoline to produce 1 gallon of ethanol. Another unknown fact is that ethanol has a finite shelf life. Part of this is due to the characteristics of ethanol. It absorbs moisture be it from the atmosphere or from direct contact with water.