Geosyntetic Clay Liner (GCL) is a prefabricated hydraulic barrier consisting of a mineral and a geosynthetic component. Nowadays, GCL is readily implemented as a liner within the landfill cover in order to prevent rainwater inflow and the emissions of landfill gasses into the atmosphere and the environment. Special consideration is given to a large amount of biogas produced in the landfill, which is most prominent at the beginning of the lifetime of any landfill. Given that those are distinct greenhouse gases, it is essential to prevent them from freely entering the atmosphere. GCL is recognized as a highly effective hydraulic barrier. However, there is a growing interest in determining the efficiency of the GCL as a gas barrier. Our research began with the characterization of samples of bentonite clays and GCL, later used for permeability testing. With the aim of more detailed analysis of the samples, chemical, physical and index properties of bentonite clays in its initial state were also tested. Based upon the granulometric analysis, with the use of a unified soil classification system, samples were classified as uniformly graded (coarse or fine) sands. Such classification refers to the size of the grains obtained during the production of GCL's and not their mineralogical composition, considering that all of the samples were bentonite clay. Solid particles’ density ranged from 2.65 to 2.86 g/cm3. The test results of index properties in all samples indicate an extremely plastic clay. The measured average values of free swell volume, water absorption and fluid loss index lead to the conclusion that samples were sodium or calcium bentonite, potentially beneficial for use in the production of geosynthetic clay barriers. To determine the optimal gravimetric moisture content of GCL for its incorporation in the landfill cover, which would ensure the lowest flow of gas from the landfill, samples with different initial gravimetric moisture content were prepared. For this purpose, a process of sample hydration on a layer of wet sand was established and applied in order to prepare bentonite clay and GCL samples for placing in the triaxial cell for gas permeability measurements. The method has proven very effective, and the analysis of the results of hydration was sufficient for precise determination of necessary time and the appropriate gravimetric moisture content of sand, for preparation of bentonite clay or GCL samples before gas permeability testing. By modifying the existing triaxial cell with an additional application of the air compressor, flow meter and data acquisition unit, the procedure for gas permeability testing was designed. Laboratory testing of gas permeability was carried out on GCL samples and bentonite clay as a separate component. By conducting a number of gas permeability tests on bentonite clay and GCL, parametric analysis was conducted, and certain properties that significantly affect gas permeability were established. Parametric analysis describes the relationship of initial gravimetric moisture content and the degree of saturation of samples with respect to the final permeability of porous media, as well as the influence of particle size distribution of the sample bentonite clays on permeability of porous media. The impact of cell pressure, during the consolidation of samples, upon the change of sample volume was observed as well. This includes the impact of increasing the cell pressure through three phases of consolidation on porous media gas permeability after each of the three phases. The effect of increasing the difference in air pressure on the permeability of porous media was observed, as was the effect of higher air pressures on the change or initiation of the flow for samples with low permeability or practicaly impermeable samples of bentonite clay or a GCL. The test procedure can be divided into several stages. The first phase refers to the sample preparation, from the aforementioned hydration used to achieve the desired initial gravimetric moisture content, up to the final sample of certain weight, height and diameter. The second phase was the installation of the sample in a triaxial cell in which the sample was isolated using a rubber membrane and the cell filled with water. The third phase was the stage of consolidation, subjecting the sample to specific cell pressure in order to achieve conditions endured by the bentonite clay and a GCL within the landfill cover. After all the preparatory stages, gas flow measurement through the sample with certain pressure difference was performed. During all the phases, data was recorded and collected. In the end, utilizing all the measured and collected data, intrinsic permeability (k), gravimetric moisture content and the degree of saturation, were calculated. By repeating the whole procedure, the method for gas permeability determination of GCLs was gradually developed, changed and improved for the purpose of developing of a more convenient, simpler and faster method. Laboratory tests enabled the identification of parameters that affect gas permeability of bentonite clay and GCLs. A functional relationship between the initial parameters (gravimetric moisture content, degree of saturation, consolidation pressure, shape and type of bentonite) and permeability was established. The relationship between the degree of saturation and gravimetric moisture content versus intrinsic permeability is linear, and it can be concluded that by the increase in the gravimetric moisture content and saturation, the sample becomes less permeable to gas. The initial stress also affects the value of intrinsic permeability, which indicates that permeability decreases with the increase of stress. The form of bentonite clay, as a separate component, demonstrates an effect on the change in permeability only in case of samples with higher moisture content. In such cases, lower permeability of powdered samples is observed in comparison to the permeability of granular once. This study describes how, based on extensive laboratory testing program, a new gas permeability test method was developed. This method is based on the use of modified triaxial cell, the improved method of sample preparation as well as the new test procedure, in triaxial device where the conditions, under which the sample is, can be controlled better than in devices and procedures that have so far been used and described in the literature.