The only feasible way to work Gc into your Poser workflow is add Gc nodes into all your models within the Material Room.  Fortunately, Bantha has uploaded a script which does that automatically, however you would need to make the necessary node connections.  Keep in mind, Poser versions prior to PoserPro rendered images linearly which, if displayed on a linear calibrated monitor, produces a very bright picture.  Since our vision cannot discern shades of gray on the brighter end of the light spectrum, a gamma correction is applied to shift the dark- and mid-tones to a point where we can recognize more detail at a sacrifice of high end values.  True black and white are unaffected on the curve. 

The problem rendering with earlier Poser versions is viewing the linear image on a gamma corrected monitor.  The image will appear darker; the temptation is to increase the intensity of your light which will, subsequently, result in specular blooms.  By applying Gc to your render, light intensity requirements will not be as demanding, meaning lower intensity values.