Yes, this value of 0.6 is the contrast index, or gamma of the straight line portion of the characteristic curve of a negative film. It can be seen in the chart in my OP.
I decided to edit this to add that the characteristic curve of a film is comprised of 21 horizontal data points, each one being 0.15 log E apart as you can see in that chart. The zone system is just a tiny center cut of that chart. The general subject of study of these curves is called Sensitometry or the measurement of sensitometric curves.
For mathematicians out there, the curve generated by all photographic materials is a cubic spline.
The curve was originally called an H&D curve for the originators of this measurement early in the history of photography.