Converting sRGB images from Color to Black and White using Luma and Luminance Under "Colors/Desaturate/Desaturate" (not a typo — "Desaturate" is one of the items under the "Colors/Desaturate" menu), GIMP 2.9 offers five options for converting an sRGB image to black and white: Luminance is...
Grayscale or Black and White • Methods of Measuring Brightness • Grayscale Mode • Desaturate • Hue-Saturation • Decompose • Channel Mixer • Coloring Monochrome Images and Making Sepia Photos • Automatic Conversion with the "Old Photo" Filter • Manual Conversion for Fine ...