I'm pretty sure it is tied to the waypoints used by AI traffic to navigate roads, and the display on the GPS screen is simply straight lines drawn between relevant nodes in the traffic structure By adding an AI pathway to this, it should register on your GPS (if I remember correctly that is)
How can I set the GPS, AI pathway in the editor menu tool? --- Post updated --- How can I set the GPS, AI pathway in the editor menu tool?
If you set the drivability to 1, or anything above zero it should work. It's in the properties. I've never used AI waypoints, so can't help there.