You should use octane level of 87 or higher. Usually an engine with higher compression needs high octane to prevent pre-detonation in the combustion chamber. The type of octane will depend on the type of engine.
The gas station is irrelevant. It's the octane rating of the fuel that's important. The higher the rating, the better quality the fuel. The G6 is capable of running just fine on 87 octane, but will run cleaner on 91 or higher.