With the widespread application of gallium nitride (GaN) in the high-power field, the heat dissipation of GaN based devices has become the main factor restricting the power density. Therefore, it is crucial to develop novel thermal management solutions. Diamond substrates with high thermal conductivity can be used to improve the heat dissipation of GaN devices. However, due to the natural lattice mismatch between diamond and GaN, the direct epitaxy of GaN on diamond substrates remains an insurmountable problem. This work achieved van der Waals epitaxy of single crystal GaN films on polycrystalline diamond substrates using a two-dimensional material/Al gradient AlGaN heterostructure as the nucleation layer between the substrate and the epitaxial layer. Among them, two-dimensional materials can effectively shield against the adverse effects caused by lattice mismatch between the substrate and epitaxial layer, while the Al component gradient AlGaN buffer layer can achieve orderly migration of Ga and N atoms, thereby accurately controlling the growth of GaN thin films. This work provides a novel approach for high-quality growth of nitrides on heterogeneous substrates. The experimental results indicate that the introduction of nucleation layers effectively eliminates the impact of lattice mismatch, thereby breaking the bottleneck of the difficulty in directly epitaxial single crystal GaN films on diamond substrates. This work provides a foundation for further improving the power density of GaN based devices.