Calcium–magnesium–alumina–silicate (CMAS) corrosion has attracted special attention in the thermal barrier coating (TBC) field. At high temperatures, when CMAS melts, it adheres to the coating surface and penetrates the interior, severely destroying the TBC. In this study, a promising CMAS-phobic and infiltration-inhibiting material, GdPO₄, on which molten CMAS is difficult to wet and penetrate, was proposed. These desirable attributes are explained by analyzing the material characteristics of GdPO₄ and its interfacial reaction with CMAS. GdPO₄ is demonstrated to have low surface energy, making it difficult for molten CMAS to wet and adhere to the surface. When in contact with molten CMAS, a double-layer structured reaction layer consisting of an acicular upper sublayer and a compact lower sublayer is formed on the GdPO₄ surface, which can effectively impede molten CMAS spreading and penetration. First-principles calculation results revealed that the reaction layer has low surface energy and low adhesion to CMAS, which are favorable for molten CMAS phobicity. Additionally, the formation of the reaction layer increases the viscosity of the molten CMAS, which can increase melt wetting and penetration. Hence, GdPO₄, which exhibits excellent CMAS-phobicity and infiltration-inhibiting ability, is a promising protective layer material for TBCs against CMAS adhesion and attack.