Here's how to do it if you already have an existing render.

Create an empty scene. Hide the ground.

Build this shader in the "Background" material. Load your render into the Image_Map.

Set your render dimensions to be the same as the original image, or a multiple with the same aspect ratio.

Adjust the Value_2 in the Pow node. Always use negative numbers. 0 means no change. The more negative, the more GC you get. A value of -.5454 is the same as GC 2.2.

Make sure you don't use Tone Mapping when you do this.

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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)