I just replaced my original OT probe with the same GRT two-wire probe. I expected the black wire to be ground, but that was wrong. I got wildly incorrect temp readings on the EMS. Once I reversed the wires everything was right again. So in my implementation Gray goes to ground and Black connects to the EMS. Not sure why others have had a different experience. It would perhaps be a good idea to do a test lash up before committing to any wire runs or cutting conductors.