I too have a Gosm and the thermo meter is really off also. I just use one of my extra thermometers in the box and smoke on. But it would be a good idea to change it but it now on the backup shelf after the new purchase of my vault 24". The thermo meter on it's door seems to be reading right for now but who knows. It would seem if they can make a thermo meter accurite to put inside the door why cann't they make one to mount in the door. Go figure
Pull off the little jam nuts on the back. Remove the thermometer. Loosen the lock nut, I think it is 10mm. Test it in boiling water or with ice and make the adjustment. Tighten the lock nut. Reinstall using the same jam nuts. You will be amazed how close it will be when you check it with a remote.