The structural and compositional dependence of gadolinium-aluminum oxide (GdAlO) for application to nonvolatile memory is investigated. An addition of Gd into AlO reduces the leakage current, which improves the erase window. The GdAlO film crystallizes into many different phases after annealing depending on the Gd percentage when the amount of Gd exceeds 49%. The crystallization of the GdAlO film causes a change in the band gap of the GdAlO film, resulting in a change of the retention properties. It is also found that crystallized GdAlO is more vulnerable to the generation of traps by electrical stress. The results indicate that careful optimization of the Gd percentage in GdAlO is necessary to utilize the benefit of GdAlO with minimum deterioration in the charge retention property.