The fault output on the GC-SDA, on pin 3 of the J1 connector is internally tied to pin 46 on the Gemini's 50 pin I/O connector. For a GV drive, pin 46 is a positional error output. For the GT drive, pin 46 is a stall detected output. Neither of these are the actual fault output.The fault output is actually on pin 43.
This means that the actual fault output is not brought out to the connectors on the GC-SDA. If you absolutely need to use the fault output then you need to make a connection on the back of the board to pin 43 and bring it out to your controller.