The polymeric hydride gallium imide [HGaNH](n), or poly(imidogallane), was isolated as an intermediate in the conversion of cyclotrigallazane, [H2GaNH2](3), to gallium nitride in supercritical ammonia. Poly(imidogallane) can be converted to nanocrystalline GaN via solid-state pyrolysis or by extended thermolysis in supercritical ammonia. Characterization by IR spectroscopy, elemental analysis, and X-ray and electron diffraction suggests that the structure of poly(imidogallane) is a layered solid with a coherent length of approximately 15 nm. The interaction of [H2GaNH2](3) and various Lewis bases leads to the isolation of [HGaNH](n) and gallane-Lewis base adducts, H3Ga.base, rather than the expected gallazane adducts, H2GaNH2.base. Experiments involving the use of ND3 resulted in the isolation of [H2GaND2](3) and suggest that the donor-acceptor complex H2Ga(NH2)(ND3) was present in solution. It was proposed that these gallazane-base adducts equilibrate via a ligand redistribution reaction and that the formation of [HGaNH](n) results from NH3 elimination from a diamidogallium hydride. The proposed mechanism is used to explain the catalytic role of ammonia in the conversion of [H2GaNH2](3) to [HGaNH](n).