As far as I know, the TeleAtlas CD's for navigation systems are not in the GDF format but in a non open, specific format developed by the manufacturer of the navigation system. I think that TeleAtlas has a product called MultiNet, which ist *very* expensive - but in the gdf format.
It might be possible that one nav. sys. manufacturer didn't develop a proprietary format and uses gdf files directly with the system or didn't chage the format much so it is easy to get the data we need.