How to adjust tones without saturating pixels, and to reveal obscured visual information using a method that can be reported as a number (versus using curves where numeric value for gamma is not shown).
How to Set Gamma in GIMP
Set as in video with Levels found under Color > Levels.
How to Set Gamma in ImageJ
Find gamma correction: Process > Math > Gamma
Type value into the Gamma box:
A lower value lightens dark areas
A higher value brightens lighter areas