Freestanding wurtzite GaN nanoprotruded microbelts with Ga2O3 core, with typical thickness 1-10 mu m, and length of few millimeters are synthesized by thermal annealing of Ga metal and subsequent reaction with ammonia at a low flow rate. They are of distinctive rectangular shape with a typical width of 10-100 mu m. Thickness of the belt is about 1/10th of the width and length up to a few millimeters. The GaN, Ga2O3 layers and the GaN-Ga2O3 interface are characterized by high-resolution transmission electron microscopy after focused ion beam sectioning of the belt. Initially, Ga2O3 nucleates after reaction with the O-2 available in the environment, and subsequent reaction with NH3 results in the formation of core-shell structure in the catalyst-free vapor-solid growth process. Having a low-symmetry phase, Ga2O3 can grow only in certain preferred directions thus controlling the final morphology of the belt. Nanoscale protrusions similar to 50-100 nm found on the surface of the belts could be an ideal system for building functional devices.