No real tips for grabbing the GPS info as I haven't done it yet, but when I was looking to do the same in my '05 TB I had come across some...Google'll be your friend there. (Just found: http://members.cox.net/onstar/ ; haven't read yet, but looks promising)
You will find you OnStar equipment under the rear seat in your truck.
Good Luck, post with any success!
Stumbled across this too: Onstar hack 2001 Yukon XL
Specifically the third post saying that it won't work after '02...so maybe not