Here is the simplest possible gamma-correcting shader. It only implements the diffuse lighting model - same as plugging the color map into Diffuse_Color and turning nothing else on.

The color map is still plugged into Diffuse_Color so that preview looks like your texture set. But the Diffuse_Value = 0, which turns off the built-in diffuse calculation.

We use a Diffuse node, which does the same thing, but we do some work before and after.

Before, we anti-gamma correct the color map, so that the colors become their true linear representation. We use the Power function for this, with a value of 2.2 for standard sRGB display gamma. This data is fed to the Diffuse node, which performs the basic diffuse lighting model calculation. The result of that is fed to another Poser function, but this uses 1 / 2.2 for the exponent, bringing us back sRGB color space (how your monitor actually works.) That is plugged into Alternate_Diffuse, which is basically a do-nothing socket. Whateve you plug in there is drawn in the render. It doesn't actually perform any sort of Diffuse lighting model.

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