Optical properties of GaN substrates are surveyed to discuss the superiority and critical issues as a substrate for device fabrication. At first, optical properties of a few hundred-micrometer-thick free-standing GaN substrates grown heteroepitaxially on (0001) Al2O3 substrates by metalorganic vapor phase epitaxy and halide vapor phase epitaxy using lateral overgrowth techniques are introduced, as they can also be used as a seed substrate for the ammonothermal overgrowth. Then, spatially-resolved cathodoluminescence (CL) spectra of GaN films grown on free-standing GaN seeds via fluid transport by the ammonothermal method are correlated with the microstructure and growth polarity. The spectral lineshape of local CL was nearly position-independent for a 4-μm-thick N-polar film exhibiting featureless morphology: the spectra exclusively exhibited a broad near-band-edge (NBE) free-carrier recombination emission with Burstein–Moss shift. Conversely, CL spectra at 100 K of a 5-μm-thick Ga-polar film having (10ī1) and (10ī2) facets with ridges originating from central pits exhibited an NBE peak at 3.444 eV and emission bands at 3.27, 2.92, and 2.22 eV, all of which showed rich intensity contrasts in the CL mapping images. The NBE peak intensity was remarkably enhanced at crests of the ridges, where the density of threading dislocations (TDs) having edge components was greatly reduced by the dislocation bending. The results encourage one to grow low TD density GaN wafers by slicing thick crystals grown by the ammonothermal method.