OnStar was a option on G6s, and a rare one at that. Since the G6 was a entry level car, and a price concious car, most do not have it. So unless you had the 3 button on your review mirror, then no. Also with OnStar you get the 1st year free, and it is activated for you, but after that it is up to you. And if you do not have a active subscription, there is nothing you can do. I know of people who broke down on the side of the road, w/o and active subscription, and could not receive help. It must be active.
That $16.95 a month doesnt seem like a lot of money now that your car is stolen, huh? OnStar also gets a insurance discount (up to 20% with some companies) and has a load of other safety features. That is somehting next time you will want to invest in. I have it on my car and I have use the lockout feature a couple of times.
Did you happen to get lo-jack installed on your car when you bought it? Some dealerships offer that and it works nearly 100% of the time.