Hexagonal GaN (h-GaN) layers are grown on GaAs(001) nominally singular substrates with the assistance of thin AlAs layers. It is revealed that the crystalline phases of GaN depend on the underlying GaAs-AlAs layered structures; h-GaN can be grown on AlAs thicker than 9 nm, while cubic GaN on AlAs less than 9 nm. Furthermore, h-GaN can be grown on 20nm AlAs covered with GaAs thinner than 8 nm. Based on those results, it is proposed that the surface electronic structures of the GaAs-AlAs layered structures, rather than the interface bonds, determine the crystalline orientations and phases of GaN.