Optical properties of GaS layered semiconductor have been characterized using temperature-dependent absorption and piezoreflectance (PzR) measurements in the temperature range between 15 and 300 K. From the comparison of optical-absorption and PzR spectra at low temperature, the gallium sulfide layer was confirmed to be an indirect semiconductor. The band gap of GaS was determined to be 2.53±0.03 eV at room temperature. PzR measurements of GaS were carried out in the energy range between 2 and 5 eV. The low-temperature PzR spectrum obviously shows three doublet-excitonic structures (denoted as series A, B, and C) presented at the energies around 3, 4, and 4.5 eV, respectively. The Rydberg constant and threshold energy of the excitonic series A, B, and C were determined. Transition origins of the A, B, and C series were examined. Temperature dependences of the interband transitions of the gallium sulfide are analyzed. The parameters that describe temperature variations of the transition energies of GaS are evaluated and discussed.