Without any images posted it would be hard to judge.  However, incorporating Gc in your PP2012 render settings will require not only tweaking your lights but re-evaluating how your lights are set up in the scene.  You must realize there are other options light IDL and self light emitting objects which help enhance Gc effects.  If you are rendering your scene set up as you would in previous Poser version, then the scene will look similar in PP2012 without Gc.  But does this scene contain many lights to compensate for the overall darkness and do those lights have light intensities equal to 100%?  If so, applying gamma correction will only wash out those exact unmodified scenes.