Here's a workaround.

I inserted an HSV node between the Refract and the gamma correction. Using that I pre-saturate the image and then gamma correct using 1.5 instead of 2.2.

This brings back a lot of the tones that get lost from using bad incoming material with GC.

And, yes, you can do these same effects in post-work. However, I like this better, because:

1) I don't like the extra step of postwork
2) I can see the true final outcome immediately, and can make adjustments in place without having to guess how that will look after I post-work.
3) If I am making an animation, the effects are already done for every frame. Postworking a movie is a lot harder than postworking a still image.

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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)