I have a new GMC-600+ and recently ran it against a sample of uranium ore certified at 1109 CPM using a GCA-07W. The CPM number is "combined alpha, beta, and gamma radiation emissions". The measurement was taken at zero distance from the sample and at the highest reading spot.
Just setting my unit over the container with the lid off is giving me a CPM of over 3000 and that's a consistent number after letting it measure over 3+ minutes. If I move the sample directly in front of the tube, it reads over 16000.
I've tried setting fast estimate to 60 seconds (defaulted to dynamic) but it still reads > 3000.
The unit seems to read background levels OK. On its own, it's usually 30-40CPM.