The present invention relates to a curtain coatable gas barrier coating composition comprising a polymer and a surfactant, wherein the polymer is selected from a group consisting of polyvinyl alcohol and polysaccharides, or mixtures thereof, wherein said polysaccharides are soluble or dispersable or suspendable in water and the surfactant is a water soluble non-ionic ethoxylated alcohol. The present invention also relates to a method for providing a substrate with a gas barrier layer by means of the coating composition, and a coated substrate 1 having at least one gas barrier layer 2 obtained by coating the substrate with the coating composition. Further, the invention relates to a packaging material comprising a coated paperboard coated with the coating composition, and a liquid package comprising such a packaging material.