Official definition;
"Gamma changes the difference between dark and light areas. Increasing Gamma makes dark areas darker and light areas lighter. It's more akin to contrast than anything.
Brightness decreases the range of lightness without changing the starting dark point."
Main purpose of a gamma slider in video games is to compensate for display differences, video cards, monitors, lighting in the room the computer is in, all effect how bright or dim the display appears.
Post process filtering;
https://en.wikipedia.org/wiki/Video_post-processing