Gallium nitride powder was prepared from gallium and ammonia at temperatures of 1000-1200 degrees C. Parameters of the crystallographic lattice as well as photoluminescence and Raman spectra were determined for the obtained powder. As a result of GaN powder sublimation, GaN single crystals of 3 x 2 x 0.2 mm were received, at temperatures 1200-1250 degrees C. Single crystals of gallium nitride were also synthesised in a reaction of gallium vapours with ammonia. Crystals up to 1.5 x 1 x 0.1 mm were obtained at the temperature range of 850-1150 degrees C, The obtained single crystals were studied by X-ray methods resulting in structure refining, mapping of reflexes and rocking curves. In addition, Raman spectroscopy studies and conductivity measurements were carried out. It was also demonstrated that the crystallographic structure of the crystals and powders was well pronounced. Homoepitaxial GaN films of several tens of micrometers were deposited onto selected GaN single crystals. The influence of grain size on the GaN sublimation process was studied. The 50-500 mu m GaN crystalline powder sublimation resulted in the growth of single crystals whose quality was significantly better in comparison to effects of the sublimation of a few micrometer crystalline powder.