As I said, I'm really busy and can't write all that is necessary to explain what you're saying wrong.

The sandwich you are looking at is empty. So of course nothing happens.

For the moment, forget about incoming color spaces.

Just make a white prop with curves with Diffuse_Value = .8.

Place it on ground also Diffuse_Value = .8

Place an infinite light any way you like but so that you can see the shape due to angle of incidence.

The light should be white, with intensity 80%.

Enable IDL.

Render with and without GC. Observe the difference.

This scenario is pure outgoing gamma, preparing to display on a computer screen. Nothing else is going on.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)