http://gimps.de/en/tutorials/gimp/picture-photo-image/black-and-white/

convert images automatically
Fastest woman use the Layers » Colors » Desaturate -tool of the Gimp to convert colors of digital images into black and white grayscale colors automatically.

This automatic method may not satisfy professional photographing woman and make colors, contrast plus brightness of the image become too dark, weak or corrupted.
Mix colors with channel mixer
Smart woman use the Filters » Colors » Channel Mixer -tool of the Gimp to convert colors of photos into black and white grayscale colors manually.
I used the following channel values to convert colors of my image into black and white grayscales: Red: 50 ; Green: 40 ; Blue: 10.

http://gimps.de/en/tutorials/gimp/picture-photo-image/index.htm#black-and-white