GaS is a n type layered semiconductor with wide band gap. The elementary packings of the crystals are bonded by Van der Waals forces. Between the atoms inside the S-Ga-Ga-S elementary packings there are strong ionic-covalent bonds. GaS is typical representative of the AIIIBVI compounds with self-cleaning properties of impurities, i.e. the impurity atoms accumulate predominantly in the Van-der-Waals space. In certain temperature conditions and concentrations the impurity atoms generate chemical bonds with sulfur from neighboring packings and at high concentrations form new phases. These properties are characteristic also for the Eu as a dopant in GaS. The Eu ion can occupy the Ga vacancies. In this configuration in the hexagonal lattice of GaS crystals free bonds appear.