Geosynthetic clay liners (GCL) are manufactured hydraulic barriers consisting of mineral and geosynthetic components. They belong to a group of geosynthetic products whose primary purpose is to seal and they have been used in many geotechnical and hydrotechnical applications, landfills and liquid waste lagoons for quite a while. They are used in landfill final cover systems to prevent the infiltration of precipitation into the landfill body and the penetration of gases and liquids from the landfill into the atmosphere and environment. Laboratory and field research and observations on regulated landfills have proven the effectiveness of GCL as a barrier for the infiltration of precipitation into the landfill body as well as the drainage of fluid beneath the landfill. Due to the presence of high concentrations of gases in the landfill body, there is a growing interest in determining the efficiency of GCL as a gas barrier. It was not until the last twenty years that the importance of this topic was recognized. In this article, current GCL gas permeability studies, the testing methods and test results of gas permeability in laboratory conditions are described.