as per edmunds.com:

The base GT1 comes with 16-inch wheels, foglights, a CD player, cruise control and power windows, mirrors and locks. The GT2 adds alloy wheels and a power driver seat, while opening up options like leather upholstery, a sunroof, an upgraded Monsoon stereo, a head-up display and satellite radio.

I have a GT2, and I bought it because I wanted leather, sunroof, and a premium stereo. While test driving, I found out it had the heads up display, a feature I enjoyed on my previous Pontiac, but wasn't sure if any other cars had it besides Pontiacs and Buicks. After I bought it, I realized that stereo is a 6CD.