At the danger of just complicating things, allow me to note that CG comes in at two levels:

1 - the computational use of colors in Poser. For example with CG = 1.0 an image with a linear varying gray pattern say black on the left and white on the right fed into the displacement channel on a flat plane geometry should show up as a straight 'ramp'. With other values of CG it is convex or concave depending on the value. This has nothing to do with the rendering of colors on your monitor, but just the way Poser uses input data. 

2 - the way the rendered (or just any) image shows up on your screen. If the CG setting is not correct, you will see the result. 
Now that I am 'at it': How does that affect CRT monitors versus LCD screen? I had understood, totally wrong of course, that CG was a concept introduced to compensate for the non-linear response of CRT screens, and that LCD flatscreens have linear behavior by themselves and need no gamma correction.