What do you mean by operating voltage of a gm counter?

When you calibrate a GM counter, you run various plots of countrate versus voltage for various energies of isotopes. Since the GM detector runs best in avalanche mode, you pick an operating point in the middle of the avalanche plateau as a compromise for the range of energies you want to be able to detect.

That is the operating voltage.

Keep in mind that, since you are operating in avalanche mode, you cannot differentiate amongst various energies, i.e. nuclides.

If you want to be able to quantify energy, you need to be running in linear mode, and the ion chamber is a better choice. Yes, the GM detector is a form of ion chamber, but it all comes down to sensitivity versus selectivity.