The gamma option on the game menu are separate from the shader option.
If you want to increase brightness you will have to open the shader options (home key) and do changes there.
Assuming you are on preset-1 it is very likely that the HDR option is causing your screen to go darker. Try to disable it or fine tune it. You can save the shader option or create a completely new one which fits your needs.
Ok i didn't know i can tweak shader but that resolved my problem i completely remove HDR i mean i unticked 2 squers and now im satisfied. Shader option window is a bit hard to navigate but i menaged with keyboard ty for a tip