The experimental and studied results of the structural, optical and nonlinear optical properties on the highly anisotropic especially un-doped and doped layered semiconductor GaSe (gallium selenide) and related crystals, InSe, GaS and GaSe-GaS (solid solutions) are overviewed. It includes also the investigation results on optical properties performed by confocal Raman and photoluminescence (PL) microscopy. Some experimental results on optical properties of GaSe nanoparticles obtained via ultrasonication and laser ablation methods are considered also.The properties of ε-GaSe is emphasized, which has one of the highest coefficient χ(2) of optical second-order nonlinearity and are crystallized into four different polytypes (ε,γ,β,δ), containing different number and arrangement of layers per unit cell. It is shown that GaSe may be considered as one of the best crystals for nonlinear applications in the IR range. More than 1 700 papers describe the physical properties of the GaAs and indicate that it is an outstanding material for applications to the teraherz (THz) spectral range. The domain structure of the crystal in connection with the Nonlinea Optical(NLO) properties is discussed by confocal Raman microscopy experiments. In spite most important physical properties of these materials are mainly investigated, further studies of optical absorption near the fundamental edge, PL, NLO properties in the IR and THz ranges as well as physical properties of their nanoparticles are necessary to understand the connection between the 2-D crystal structure and the physical properties. It is known that the nanoparticles of GaSe and GaSe- type crystals are highly interesting because they have a single tetra layer structure consisting of covalently bond –Se-Ga-Ga-Se- tetra layers. Some of GaSe-type crystals have band gaps in the range of 1.2-1.5 eV (InSe, GaTe) which make them and their nanoparticles suitable for photovoltaic applications.