The valence band density of states of GaS has been investigated by X-ray photoelectron spectroscopy and the spectrum has been interpreted on the basis of a single layer tight-binding calculation. Our two-dimensional approximation seems largely appropriate to reproduce the main experimental features of the valence band density of states of GaS, as previously found in the case of InSe. It can also explain to some extent the fine structure in the ultraviolet photoemission spectra recently measured with the synchroton radiation.