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. Changing gamma differs from 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. Similarly named controls can affect different images. Check whether you are adjusting a stream or the display output.

See Image Adjustments for the display controls.