Gamma settings are not working in Amnesia: The Dark Descent.

Thread in steam for linux discussion thread:

http://steamcommunity.com/app/221410/discussions/1/882965239646730976/#c846938351169608267

There is a link to explanation from Frictional Games forums in last post:

http://www.frictionalgames.com/forum/thread-3760.html

Inability to set gamma on ATI cards or gamma too high on ATI cards

It seems the ATI drivers are not supporting the X method used to set the gamma that is used by SDL_SetGamma, a workaround is to use the xgamma utility that should be available with your distribution to adjust the gamma.

Thank to devs for their hard work.