Gamma

A gamma adjustment changes the transition through the middle tones between black and white. More generally, gamma describes a nonlinear relationship between image values and displayed brightness. This helps explain why changing gamma can alter the look of an image without being the same as changing the monitor's backlight. Apple’s gamma-adjust documentation

In BetterDisplay, pay attention to which image you are adjusting. A gamma filter on a captured stream changes that stream's image. A display image adjustment affects the selected display's output instead.

Both are distinct from hardware Brightness. If you are comparing adjustments, keep their scope in mind rather than expecting similarly named controls to affect everything on screen in the same way.

See Image Adjustments for the display controls.