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In this work the n-GaN(1000) surface is used as a source of nitrogen atoms in order to obtain niobium nitride film by a surface-mediated nitridation technique. To this end, the physical vapor deposition of the niobium film on GaN is followed by sample annealing at 1123 K. A thermally induced decomposition of GaN and interfacial mixing phenomena lead to the formation of a niobium nitride compound, which contains Nb from thin film and N atoms from the substrate. The processes allowed the obtaining of ordered NbN_x films on GaN. Structural and chemical properties of both the GaN substrate and NbN_x films were studied in-situ by surface-sensitive techniques, i.e., X-ray and UV photoelectron spectroscopies (XPS/UPS) and a low-energy electron diffraction (LEED). Then, the NbN_x/GaN surface morphology was investigated ex-situ by scanning tunneling microscopy (STM).