I didn't see any difference when I changed it from 9 to 1 and 19, seems like the change didn't have any effect. Is this a bug, or am I missing something?

Only MacOS X has a functional Gamma Correction API. So, that option only impacts the Mac.

The game does not offer internal gamma correction. We may eventually add more options to the engine, that dig into HDR and tone mapping, and add our own gamma options at that point.

For now, space fungus, it may be best to just tweak your display or driver's settings.

Gamma correction was mentioned in your manual, but I didn't see it in the game settings, so that's why I went to the config file. Understood.