This information is taken nearly verbatim from Robert Berger's web
document

  Why Do Images Appear Darker on Some Displays?
  An Explanation of Monitor Gamma

(URL http://www.cs.cmu.edu/afs/cs.cmu.edu/user/rwb/www/gamma.html)
Those interested in further information about monitor gamma and gamma
correction should start by reading that document.

The gamma measurement image (gamma-test.gif) included in the xearth
source distribution allows you to directly estimate the gamma of your
display system. After loading the gamma-test.gif image for viewing
using your favorite image viewer (e.g., xv, xloadimage), stand about 6
feet away from your monitor and decide which column of the image comes
closest to having equal brightness in the top and bottom halves. The
number under this column is the gamma of your display system.