should i run 87 regular or 89 in my grand prix v6 3.8 gt?

You can run what ever you want, but the higher the octain rating, the cleaner your engine stays, and performs better.

87 should be fine but if it pings then use a higher octane gas.

Run whatever the manual says to run. My '03 says to run mid grade which is 87 to 89 here in Canada. Yours is quite likely the same. Modern cars are are carefully engineered. Things like octane ratings affect the operating temperature of the engine and that temperature is the best temperature for optimizing your car's performance. Different fuel can change that temperature. The computer reads that, sees a fault and starts adjusting things to compensate. The end result can be a car that doesn't run as well or as efficiently. GM spent a gazillion dollars determining that that engine works best with that particular fuel. The few cents a gallon you will save by using regular fuel is a false economy. By the same token using a higher than specified octane fuel can have similar adverse effects and is money thrown away.