In spite of the progress in GaSe growing technology (high-quality crystals grown by the latest technology are characterized by low values of optical losses with the absorption coefficient values lying below 0.1 cm−1) the work continues on improving crystal properties for nonlinear optical applications. This paper presents the results of investigations on the influence of annealing in a sulfur atmosphere and in a vacuum on the properties GaSe single crystals, grown by the Bridgman method from the melt. The objective of this work was to study the possibility of intercalating GaSe with sulfur from the gas phase, and to compare the influence of doping with sulfur from melt and vapor phase on the structure and properties of GaSe crystals. Three series of annealing experiments have been conducted at temperatures 773, 923, 1073 K and the samples obtained have been studied by Hall effect, photoconductivity, optical absorption, microhardness measurements and X ray diffractometry. The results obtained are explained by assuming the intercalating of sulfur to the interlayer space and substitution of Se with S.