Gamma just another word for Curve - apple M in photoshop
Mostly a normal gamma lifts the mids
Knee is a flattening of the top of the curve in an effort to stop highlights clipping
IMO Knee is the devil and give the video look of plastic skies, or plastic skin if your skins enter the knee zone.
Now. there is also some gamma stuff for different screens, basically AFAIK CRT screens crush the darks - so video people apply a standardised curve to counter this, lifting the darks