Gamma adjustment doesn't work

[Prophet5]

Sorry for bumping the topic, but is there anything I could try?

The config you want is this one:
/home/<username>/.frictionalgames/Amnesia/Main/main_settings.cfg

It has the gamma setting (at least for me; not using steam if that helps).

Incidentally Gamma works perfectly fine on my system. Just upgraded to OpenSUSE 12.3 with KDE SC4.10 AND oxygen-transparent theme with a whole bunch of composited goodies active (nVidia drivers though). Having said that, KDE has an option to disable compositing for fullscreen applications; which I have enabled. Maybe Gnome has something similar that is enabled by default and LXDE doesn't hence why it works in Gnome... I should probably test that right now... Nope, still works.