The optical absorption behavior has been studied for annealed and illuminated thermally evaporated amorphous GaS thin films. It is observed that photo- and thermally induced effects result in an increase of the optical band gap and the slope parameter of the Tauc plots for these films. The thermally and photo-induced bleaching are ascribed to the suppression of the density of dangling and homo-polar bonds in these films and/or the relaxation process of the disordered state to another relatively high ordered one.