Quote - 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.

 No, it's not the lights... And the difference is quite easy to explain:
When GC is enabled, the inverse of the chosen gamma correction is done to all image maps *) to linearize them. After rendering the image is gamma corrected. With material based GC the step to linearize the data is not done.

*): according to the gamma setting chosen for the image map (it defaults to the same as the render setting, but needs to be set to 1 for all maps that should not go through that process, like displacement, bump,...).