The metal stop laser drilling of GaN-on-GaN devices is demonstrated using a UV sub-nanosecond laser as a light source. By monitoring the Bremsstrahlung emission at the drilling point, metal stops with a precision higher than 1 μm are realized for vias with a depth of 100 μm. From in situ laser-induced breakdown spectroscopy measurements, it is shown that endpoint detection is realized with high signal-to-noise ratio owing to the difference in the emission process between the strongly excited semiconductor and the metal. Herein, a through-substrate electrode with a resistance of less than 5 mΩ on GaN-on-GaN high-electron-mobility transistor (HEMT) wafers is demonstrated. The fabrication of through-substrate electrodes by this technique provides a simple process that does not require lithography or other complex processes. This process is expected to be useful in the fabrication of future GaN-on-GaN devices, including very thin GaN-on-GaN HEMTs.