The Gamma Correction attributes are deprecated and no longer appear in Maya 2017. You should be using Maya's native Color Management settings. More information about using Color Management with MtoA in Maya 2017 can be found here.
Gamma correction controls are helpful for users to make sure they are working in radiometrically linear space (also called 'linear workflow'). Linear workflow ensures that all of the elements interacting in a scene (light colors, textures and shader colors) correspond linearly with actual amounts of light so that the renderer does proper light computations in a physically accurate way. This is needed since light works linearly in the physical world (if you double the number of photons, the color values are also doubled).