I assume it comes with lights?

It's likely that the lights themselves use shades of gray in their color chips - a no-no when trying to switch between non-GC and GC. It changes the light levels.

To recover the original lighting level (which you may want to lower again later) you need to read the values in the lights.

Given original light:

RGB X, X, X Intensity Y

You would switch to

RGB 255, 255, 255, Intensity (X / 255) * Y

The reason is that enabling GC does not just do gamma correction at the end. It converts all incoming material to linear values - this involves anti-gamma correcting incoming colors. The point of enabling GC is linear workflow, not gamma correction. GC as a final step in linear workflow somehow has come to be the name of the whole thing. This causes a great deal of confusion. The checkbox should be "Linear Workflow" not "Gamma correction".

So - suppose you had a light with RGB 180, 180, 180 and Intensity .7. The effective illumination with GC (Linear workflow) enabled is:

((180 / 255) ** 2.2) * .7 = .325

whereas the intention was

(180/255) * .7 = .494

So in that case you'd switch to RGB 255, 255, 255, intensity = .494 or 49.4%.

Then you have the same amount of light as before.

 

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