The work demonstrates the dominant role of nitrogen flux rate on GaN nanostructure formation on bare Al2O3(0001). In nitrogen rich conditions, wurtzite c-oriented GaN nanowall honeycomb network is formed as strain relaxation pathway of nucleation at edge dislocations. A specific nitrogen flux rate in a plasma assisted molecular beam epitaxy growth is necessary for fixed Ga flux and substrate temperature to form columnar self assembled nanostructures. It is argued that kinetically hindering diffusion of Ga adatoms and the low sticking coefficient of r and m planes of nanowalls promote 1-dimension nanocolumn formation at screw dislocations formed at the GaN-Sapphire interface.