Gamma settings are not working in Amnesia: The Dark Descent.
Thread in steam for linux discussion thread:
There is a link to explanation from Frictional Games forums in last post:
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.