Convenient single-step N-for-As metathesis reactions of gallium arsenide GaAs with ammonia NH3 at temperatures in the range 650–950 °C for 6–90 hours afforded in this oxygen-free system high yields of pure nanocrystalline powders of the wide bandgap semiconductor gallium nitride GaN. High energy ball milling via noticeable amorphization of the monocrystalline cubic GaAs substrate enabled complete ammonolysis and nitride preparation at lower temperatures and shorter times relative to manual grinding. Under the applied conditions, all by-products were removed as volatiles affording pure GaN nanopowders. Reaction-controlled average crystallite sizes ranged from a few to a few tens of nanometers. When compared to the related ammonolysis reactions of cubic GaP and cubic GaSb which yielded, respectively, either the hexagonal polytype only or mixtures of mostly hexagonal with some cubic GaN polytypes, here, the nitride could be made both as solely hexagonal and as a mixture of two polytypes in a wide composition range. All this supports diverse reaction pathways which were found to be closely correlated with substrate grain size characteristics. The ball milled fine GaAs particles afforded only hexagonal or hexagonal GaN-enriched mixtures pointing to predominantly thermodynamic reaction control. Under similar conditions, the manually ground coarser GaAs particles yielded cubic GaN-enriched mixtures, instead, consistent with prevailing topochemical control.