Just for the record, in case anyone else's sees this thread and wants to give GDL90 a go, I want to be really clear that with GDl90 you currently get *No* reporting of bearingless traffic. So no range rings, no mode C or S traffic warnings via your navigation software. (You can still get them from audio warnings direct from the PilotAware).
GDL90 doesnt have a specific way of communicating bearingless targets and hence Skydemon doesnt know how to expect the data. Hopefully that can change in the future with an agreed (informal or otherwise) addition to the standard.
Cheers
Kev