In the early days the term gamma was used to describe curve slope, and for a straight line part of the curve it is an angle found by dividing the density range of the negative by subject brightness (or subject luminance) range. Most modern films have a limited straight line section so film makers today use other terms. G-bar is Ilford's term for Average Gradient, Kodak uses CI, or Contrast Index. G-Bar and CI are determined in slightly different way but produce similar results, depending on curve shape.
N, or Normal contrast, is a Zone system term that must be associated with the contrast requirements of negatives for a given process. Film makers don't make that determination, but do provide G-Bar and CI charts that can be used to determine how much contrast one will get with a given film, developer dilution and method, and time of development. These charts can be very useful, if you know how much average gradient or contrast index you need for a given process, and type of lighting.
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