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We present an overview of the current state of the literature and research performed by the authors of the present paper on the experimental and theoretical results on the structural-, optical-, nonlinear optical (NLO)-properties (including two-photon absorption (TPA) and the terahertz (THz) range of spectra) and practical applications of a highly anisotropic Gallium Selenide (GaSe) semiconductor with emphasis on the ɛ-GaSe. Physical properties of ɛ-GaSe are important to researchers and designers developing different devices by using this material. This crystal possesses an outstanding NLO properties: high optical birefringence Δn ∼ 0.3 at 700 nm; high transparency range (0.7−18.0 μm) with low absorption coefficient (α ≤ 0.3 cm−1); very high nonlinear susceptibility χ(2) (d 22 ≈ 86 ± 17 pm/V, corresponding to (2.0 ± 0.4) × 10−7 esu) that is used for phase matched second harmonic generation (SHG) in a wide transparency range; high power threshold for optical damage; possibility to perform optical frequency conversion under phase-matching conditions in the near- to mid-IR and THz range of spectra, etc. The domain structure of crystal in connection with the NLO properties is discussed as studied by confocal Raman microscopy experiments. Perspectives for future research of GaSe are considered in the present article, which does not pretend to be one reflecting all existing papers on GaSe crystal and discussed subjects.