Nice to see another Lowrance 2000c user!
Like you, I'm using the 2000c to provide GPS data to the D100.
The GRT Mini-X would do the trick as well using its NMEA output format. BUT you would have to use the flight planning function within the Mini-X in order to get the appropriate data included in the NMEA output. In essence, the autopilot and D100 need to know where you want the airplane to go, where you are now, and what the current direction of travel of the airplane is (GPS-derived velocity info). The GRT will provide all of that good stuff but only if you have an active flight plan with waypoints etc alive and running in the Mini-X.