Now that I am playing with Poser Pro and rendering with GC, I can see a significant difference between the results I get with that versus rendering with material GC. Different doesn't not imply any sort of qualitative appraisal. I don't feel I have a true sense for renderer GC yet. But in my as yet limited experience, one does not obtain similar results rendering with material GC (set to 2.2) and renderer GC (with same material's GC all set to 1). Must be the lights.

After all, that is an important part of the whole equation, and one that I only ever addressed in IBL before (I linearised the probe image only). Since main light was set to 100% intensity, RGB (1,1,1), no GC was needed there.

Same lights. Same scenes. Different results. Before I start posting images that might have unknown variables unaddressed, I'm going to run some extremely simple test renders on basic, basic materials, and work my way up.

Poser Pro is truly a pro tool, and GC in this setting is a pro feature. I can see that now. It is easier starting with an un-tweaked shader setup and applying render GC, but I have so many saved P7 material-GCed scenes that I'm now bringing into Poser Pro. Very interesting results... and jeez, the quality of the render is PHENOmenally better. Not the GC bit, but just the quality of the image in terms of tonal gradients and like that.

What did they do to the renderer?

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks