Gallium nitride (GaN)-based vertical-cavity surface-emitting lasers (VCSELs) have attracted considerable attention owing to their low power consumption, small beam divergence, circular beam profiles, and compatibility with high-density two-dimensional arrays. These advantages render them highly promising candidates for the applications of laser projection, retinal scanning and visible light communication. In this work, a long-cavity planar GaN-based VCSEL incorporating dual dielectric distributed Bragg reflectors was fabricated using metal bonding and laser lift-off processes. The current confinement apertures with diameters of 12 and 15 μm were employed to reduce the increased diffraction loss associated with the extended cavity length. Owing to the superior heat dissipation capability of the long-cavity structure and the larger current aperture, the device exhibited excellent thermal stability under continuous-wave current injection at room temperature, with a measured thermal resistance of 349.9 K/W, representing the lowest value reported to date. Under optical pumping, a pronounced lasing peak was observed at 435.7 nm, with a full width at half maximum of approximately 0.45 nm. Under pulsed electrical injection, the emission spectrum gradually evolved from multiple longitudinal modes to single-mode operation as the injection current increased. The threshold current densities ranged from 45.3 to 107.9 kA/cm². The lasing spectrum exhibited a full width at half maximum of about 1 nm with slight asymmetry. Further far-field pattern measurements revealed distinct double-lobed beam profiles, indirectly confirming the presence of higher-order transverse mode superposition in the electrically pumped lasing spectrum. These results demonstrate that the proposed long-cavity device architecture constitutes an effective strategy for thermal management and diffraction-loss reduction, thereby providing a viable pathway toward the realization of high-performance GaN-based VCSELs with enhanced efficiency, stability, and practical applicability.