What does the gamma histogram represent compared with the regular brightness/RGB histogram?

The “gamma” histogram is typically showing luminance (or luminosity), not a simple sum of RGB values.

A regular RGB histogram is based on the red, green, and blue channel data. Depending on the display mode, it may show each channel separately or combine them. A luminance-style histogram instead weights the color channels according to perceived brightness, because our eyes are more sensitive to green than to red or blue.

So two histograms can look quite different even for the same image:

• RGB histogram: reflects pixel values in the color channels
• Gamma/luminance histogram: reflects perceived image brightness after channel weighting

That’s why areas with strong green content can affect the luminance histogram more than equally bright-looking red or blue areas.

In short, the gamma-adjustment histogram is useful for judging tonal distribution as it relates more closely to how bright the image appears, while the RGB histogram is more useful for evaluating channel-specific clipping or color balance issues.