I hope I didn't miss anything 
I've commented the code I've previously posted (both in repeatedly_execute and in on_key_press) and followed your instructions for the slider. So the only code I have that deals with Gamma is the sldGamma_OnChange function.
Still no results (at least, here on Win8) :\
Just for the sake of testing, after that, I even tried putting a System.Gamma = 0 in the game_start function and still nothing seems to happen.

I've commented the code I've previously posted (both in repeatedly_execute and in on_key_press) and followed your instructions for the slider. So the only code I have that deals with Gamma is the sldGamma_OnChange function.
Still no results (at least, here on Win8) :\
Just for the sake of testing, after that, I even tried putting a System.Gamma = 0 in the game_start function and still nothing seems to happen.