Gallium sulfide (GaS) is a highly promising two-dimensional layered semiconductor owing to its remarkable thickness dependent electronic and physical properties. In this article, we perform a comprehensive ab initio study of lattice dynamics, mode symmetry assignments, polarized Raman and infrared (IR) reflectivity spectra of GaS system. Polarized Raman spectra are obtained for different light polarization set-ups of incoming and scattered light. The frequencies of all allowed vibrational modes at the zone-centre are calculated and symmetry labels are assigned. Furthermore, the variation of frequencies & intensities of Raman/IR active modes of ultrathin GaS films (few layers) as function of film thickness is studied. In addition, we also explore the nature of weak interlayer coupling in GaS. The weak forces between the GaS layers are usually assumed to be due to interlayer van derWaals (vdW) interaction. However, this assumption has not been reasonably explained in reported experimental studies. Our study strongly suggests that weak interlayer interactions in GaS may be primarily electrostatic (Coulomb) in nature and therefore the contribution of vdW interactions to layer-layer coupling and lattice dynamics may be significantly lower than that of electrostatic interaction. The suggested nature of interlayer coupling in GaS and related III-VI semiconductors may have important implications in determination of their various physical properties. © 2021 IOP Publishing Ltd Printed in the UK.