There are significant differences in the extent of impurity incorporation on different crystallographic directions of GaN microstructures, and the impurity-related deep energy level behavior will have a significant impact on device performance. However, a comprehensive understanding of the effect of lateral growth on device performance has not been achieved due to the lack of comprehensive spatial distribution characterization of the optical behavior and impurity incorporation in GaN microstructures. We present a comprehensive study of the optical behavior and growth mechanism of self-assembled GaN microdisks using nanoscale spatially resolved cathodoluminescence (CL) mapping. We have found a clear growth orientation-dependent optical behavior of the lateral and vertical growth sectors of self-assembled GaN microcrystals. The lateral growth sector, i.e., the { $10\overline{1}1$}-growth sector, forms six side facets of the microdisk and shows significant near-bandgap emission (NBE) and weak deep energy level luminescence. Cavity effect enhanced emission was found for the first time in such a truncated hexagonal Na-flux GaN microdisk system with an ultra-smooth surface (R_a < 0.7 nm) and low stress. The self-assembled microdisk shows significant ultraviolet (UV) lasing action (main lasing peak wavelength 370.9 nm, quality factor 1278, threshold 6 × 10⁴ μJ/cm²) under pulsed optical pumping. We believe that the appearance of UV lasing action may be related to the light limitation on the six side facets of the lateral growth of the GaN microdisk, the high structural quality, the low content of deep energy level defects, the low surface roughness, and the low stress.