Growth nucleation and evolution of morphology of GaN on (001) GaAs is investigated as a function of the N2H4/Ga flux ratio. The use of hydrazine allows us to reach high flux ratios without causing any damage to the epitaxial layer. Epitaxial GaN is purely cubic but shows growth anisotropy dependent on the flux ratio. GaN layers grown at low flux ratios show three-dimensional nucleation and no preferential island orientation. With higher flux ratios, the nucleation rate increases, the surface becomes smoother, and the growth anisotropy markedly increases. The growth morphology reflects the surface anisotropy of the underlying GaAs substrate.