Correct midtone brightness with a power curve instead of a linear shift. Gamma remaps every value between black and white along the curve output = input^(1/γ), so a photo that scanned too dark or rendered too flat can be rebalanced without clipping either end of the histogram.
Midtones, not endpoints
The defining property of a gamma curve is that 0 stays 0 and 255 stays 255. Everything between bends. At γ = 1.4 a middle gray of 128 rises to about 160, while a shadow value of 20 only moves to 41 and a highlight at 230 barely shifts. That is why gamma is the standard fix for images that look dark overall but still contain true blacks and whites: a brightness slider would wash them out, gamma redistributes them.
Choosing a value
Values above 1.0 brighten midtones; values below darken them. Screenshots and renders from software with a wrong transfer curve usually need a coarse correction such as 0.45 or 2.2, matching the encode or decode step that was skipped. Photographic fixes stay subtle, typically inside 0.8 to 1.5. If a correction beyond that range looks necessary, the problem is more likely exposure, and the exposure adjustment tool gives finer control over highlights.
Per-channel correction
The red, green, and blue sliders apply an additional gamma on top of the master value, one channel at a time. Because each channel keeps its own endpoints anchored, this removes midtone color casts that a global tint would overcorrect: an aged photo with orange midtones takes a small red gamma reduction and keeps its neutral highlights. Channel values multiply with the master, so master 1.2 and red 1.1 produce an effective red gamma of 1.32.
The remap
The correction is computed once as a 256-entry lookup table per channel and applied to every pixel, so the result is identical at preview size and full export resolution. Alpha is untouched; transparent PNG input downloads with its transparency intact.