A solid electrolyte layer or body capable of being co-fired with an insulating ceramic substrate or body to be used as a gas sensor laminate. The gas sensor laminate is preferably composed of an alumina substrate (1) and an oxygen-ion conductive solid electrolyte layer (6) bonded to the alumina substrate (1) by firing. The oxygen-ion conductive solid electrolyte layer or body is formed from partially or wholly stabilized zirconia, which layer contains alumina in an amount of 10% to 80% by weight, particularly 30% to 75% by weight. In this manner, another ceramic layer; particularly, a ceramic layer (7) of alumina can further be bonded to the oxygen-ion conductive solid electrolyte layer. The relative density of the ceramic layer can be 60% to 99.5%, preferably 80% to 99.5%.