First off, WELCOME!
Now, If you buy a replacement OEM stereo you will have to get it programmed by the dealer for you VIN so it'll work in your car, otherwise no dice.
You can easily get an aftermarket head unit. But you need to get a Metra Dash kit.
here's one on amazon: http://www.amazon.com/Metra-99-3303-Vehicles-Integrate-Diagnostics/dp/B000G7SGO4/ref=sr_1_2?ie=UTF8&qid=1358940658&sr=8-2&keywords=pontiac+g6+dash+kit
If you get that kit and install it in the dash, you'll then have room for a smaller single din stereo. Which is fine, But some people want the larger double din units with a touch screen and everything, Those will fit and can still work, but then you're left with the metra kit, Some members have relocated that to the center console or the glovebox, So it's there and still usable, but out of site.
There's actually a user on here who is right now selling a single din stereo with the metra dash kit.