Abstract: The present invention relates to a curtain coatable gas barrier coating composition comprising polyvinyl alcohol and a surfactant, in which 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 having at least one gas barrier layer 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.