Van der Waals (vdW) dielectrics are extensively employed to enhance the performance of 2D electronic devices. However, current vdW dielectric materials still encounter challenges such as low dielectric constant (κ) and difficulties in synthesizing high-quality single crystals. 2D rare-earth oxyhalides (REOXs) with exceptional electrical properties present an opportunity for the exploration of novel high-κ dielectrics. In this study, for the first time, the synthesis of a series of van der Waals layered gadolinium oxyhalides with thicknesses down to monolayer through a space-confined vdW epitaxy approach and demonstrating their application as a single-crystalline gate dielectric is reported. It exhibits a remarkable relative dielectric constant exceeding 17 and an impressive breakdown field strength of 13.5 MV cm