The downscaling of complementary metal-oxide-semiconductor technology has produced breakthroughs in electronics, but more extreme scaling has hit a wall of device performance degradation. One key challenge is the development of insulators with high dielectric constant, wide bandgap and high tunnel masses. Here, we show that two-dimensional monocrystalline gadolinium pentoxide, which is devised through combining particle swarm optimization algorithm and theoretical calculations and synthesized via van der Waals epitaxy, could exhibit a high dielectric constant of ~25.5 and a wide bandgap simultaneously. A desirable equivalent oxide thickness down to 1 nm with an ultralow leakage current of ~10