Porous and sub-micrometer tubes made of textured GaN nanoparticles have been synthesized by an in situ chemical reaction and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) and Raman spectroscopies. The in situ reaction involves thermal decomposition and nitridation of ID gallium oxyhydroxide (GaOOH) at temperatures in the range of 700-900 degrees C. The 1D shape of the precursor GaOOH is maintained in the resultant GaN tubes. The GaN nanocrystals (estimated to be about 15 nm in size) are found to be highly oriented with respect to each other in the tube structure, with the [110] GaN direction parallel to the tube axis. The growth mechanism of the tube structure has also been studied. beta-Ga2O3 is found to be an intermediate phase between the starting GaOOH precursor and the final GaN product. The growth mechanism involves decomposition of GaOOH, which produces beta-Ga2O3 tubes with hollow interiors, and nitridation of beta-Ga2O3, which leads to growth of textured GaN nanocrystals. Based on the growth mechanism, tubular structures with either quasi-circular or rectangular cross section are selectively synthesized by controlling the heating rate and calcination temperature. This in situ chemical reaction method provides a new route for synthesizing 1D hollow nanostructures.