Contrast Threshold refers usually to the smallest amount of contrast needed to detect a pattern, so lower threshold = higher quality anti alias .

Gamma correction is used to convert rgb linier space into an digital video signal. The gamma helps keep the color space as expected.

Practically though in CG it can be used as a function that will increace or decrece brightness in the midrange values of an image while leaving darks dark and lights light.

A gamma correction node will convert hsv color space to rgb space.

Gamma has associations with HDRI as well, since hdri is in linier space and not made for viewing a tone mapper can basically be used to bring the range into viewable space, using gamma correction as a controlling function.

Mental ray supports a per pixel and lens or sample based tone mapper.

Hope that helps.