Unintentionally doped impurities formed in the microstructures of free-standing GaN grown with facets were studied using confocal magneto-photoluminescence (PL) microscopy. Donor-bound exciton related peaks in PL spectra and their magnetic behavior allowed us to distinguish typical donor impurity atoms, such as silicon and oxygen. Combining this technique with confocal microscopy also revealed the spatial distribution of the impurities. The results showed that angled facets tend to incorporate oxygen. Moreover, even facets angled at a few degrees with respect to the (0001) surface cause a noticeable change in oxygen incorporation on the order of 1 × 1016 cm−3.