Most of this discussion has been at cross purposes almost from the start.
On the one hand, trying to explain, in response to the OP, the simple, plain fact of what GC is.
On the other hand, presenting good and bad renders as reasons to either use it or not use it.

GC does nothing more or less than enable us to use physics-based maths in our shaders. No GC means that shader lighting calculations are not based on physical reality. But that simple fact in no way implies any pressure on anyone to use it. There are other ways to compensate for these inaccuracies.

The fact remains, though, that many ugly Poser renders are ugly simply because the contrast and/or levels are all wrong. It is in such circumstances that those people who recognise the symptoms are likely to recommend GC as an appropriate way to improve things.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)