So I am trying to change the gamma values and brightness in my game. I think it has something to do with IDirect3DDevice9::SetGammaRamp function but I am not exactly sure how to use it. Anyone has any input on this?
The Win32 SetDeviceGammaRamp and GetDeviceGammaRamp calls are also options for you; they will work in windowed modes (but will affect the entire screen rather than just the game window) and are somewhat better documented than the D3D versions.
You can also use render to texture and just brighten the texture while drawing.