The fundamental absorption edge of the gadolinium trihydride has been studied by performing optical transmission measurements at temperatures ranging from 298 K to 353 K. Urbach tails and the photon absorption spectra give information about the exciton dynamics, distinguishing between free and trapped excitons in the lattice. Our experimental results can be accounted for in terms of the theory of Toyozawa and co-workers, which ascribe the Urbach tail to the momentary localization of excitons due to phonon interaction.