Ok so you make a gps driver, piece of cake. Im sure there is serial to usb drivers on debian. Now you have 2 numbers in your computer, latatude and longitude. Easy as you say. The trick is doing something with them.

How do you get maps?
How do you render maps quickly and efficently?
How do you make routes?
How do the routes know where one ways are?
How do you know where addresses are on the maps?
How does it determine when you are off course?
How does it determine where POIs are?
How do you make the interface pretty?
How do you make the speech engine?
How do you do all this on an xbox where the developer support community is pretty small?

Thats at least 1,000 hours of coding/testing there. If time were worth $15 an hour that project costs $15,000 dollars.

Its not easy making that software. And its even harder to port one from windows to the xbox. And the demand is slim to nil - xbox is going out of style and the only people that want xbox gps software is 20-30 mp3car people like you who aren't willing to work at a part time job an extra 8 hours to get superior hardware.

Sorry if I seemed hostile.