Light emitted during the development, of electron avalanches in a gas electron multiplier (GEM) can be efficiently used for optical readout of the detector. We report on measurements of total light yields in Ar/CF4, He/CF4, Ar/TEA and Xe/TEA mixtures. Information on the energy resolution obtained with low-energy X-ray photons is also,presented. The spectral distribution of the light produced in the GEM is analysed and the mechanisms associated with the process of light production are considered. (C) 2003 Elsevier Science B.V. All rights reserved.