Two-dimensional (2D) layered III-VI semiconductors such as GaSe have attracted a lot of attention in recent years. Bulk GaSe consists of stacks of layers held together by weak interlayer interaction generally assumed to be of van der Waals type. However, proper justification of this assumption has been lacking in the reported studies. In this paper, we explore in detail the interlayer coupling in GaSe by studying lattice dynamics using first-principles density functional theory. Our study strongly suggests that contrary to common assumption, the contribution of Coulomb interaction in interlayer coupling can be significantly higher than that of van der Waals interaction in GaSe and other similar 2D layered semiconductors. The suggested predominance of electrostatic over van der Waals interaction in interlayer coupling may have important implications for various physical properties of GaSe and related layered semiconductors. Further, we study polarized Raman spectra, infrared (IR) activities, mode symmetry assignments, and Born-effective charge tensors for bulk GaSe polytypes (β, ϵ, γ). The Raman mode intensities are calculated for different light polarization setups and signature Raman and IR active modes are identified for each GaSe polytype (structure). In addition, the influence of film thickness and strain on Raman and IR mode frequencies and intensities of GaSe are explored and compared with available experiments. © 2021 American Physical Society.