Then why GC in Poser?

Because we want to brighten up the shadows, to compensate for the lack of radiosity, the lack of scattering in the air (dust particles etc) and more like that. So what does Poser do?

For a regular coloring C, and an effect of shading and shadowing S, and a gamma value g, it first adjusts all images and colors to C^g, then it applies (renders) the S to C^g * S, and then it readjusts the render result to ( C^g *S ) ^1/g = C * S^(1/g). And since G>1 and S 0..100%, S increases and all shading and shadowing brightens up,  while original colors go un-adjusted.

(Un)fortunately, Poser can chosen to use the GC function from image engineering to do so, and (un)fortunately, the industry standard value of g=2.2 presents very nice results in shadow brightening. So now everyone confuses the Poser GC with the image-engineering GC. Bad luck. They could have used another mechanism instead, with similar results, but they didn't.
Unfortunately too, Poser applies its GC to all kinds of brightness dimming. Not only shading and shadowing, but also semi-transparency, limited reflection and the like are altered by it. While it should not, as these do not require any adjustment for the lack of environmental lighting abilities in Poser.

Other consequences are:

 - when Poser is part of a larger workflow, and componenents (difusse, shadows, ...) are obtained separately (or as PSD layers in the export), then one should not apply GC but make the adjustments later on. As it's GC which de-linearizes the render result. It's like image compression (save as JPG) which should be postponed to the latest stage as well.

 - when IDL is used, radiosity is 'ON' and one does not need the large adjustments from gamma = 2.2 as part of the problem is solved in IDL already. Far lower values - combined with reduced shadow intensities - produce more realistic results. And have far less effects on transparency and raytracing as well.

= = =

to all: my opinion, please do PROVE me wrong so I can learn from it, but do note I'm not fooled or convinced that easily for being into computer HW, SW, images, optics and physics for some decades now. As is BB, by the way.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though