A method of producing a gas barrier laminate is disclosed that comprises: preparing a laminate comprising a plastic substrate, a gas barrier layer formed from a gas barrier layer-forming coating material comprising a polyvinyl alcohol and an ethylene-maleic acid copolymer, and a polymer layer comprising a metal compound of a bivalent or higher metal, wherein the polymer layer is laminated to at least one surface of the gas barrier layer; and heat treating the obtained laminate in the presence of water. In addition, another method of producing a gas barrier laminate is disclosed that comprises: applying a gas barrier layer-forming coating material comprising a polyvinyl alcohol and an ethylene-maleic acid copolymer, either directly onto a plastic substrate, or onto a plastic substrate with an undercoat layer disposed therebetween, and then conducting a heat treatment; and heat treating the obtained laminate in the presence of water comprising a metal compound of a bivalent or higher metal.