With the adoption of wide-bandgap (WBG) semiconductor devices in high power density applications, the thermal dissipation of this type of power modules has become a major challenge. In the power conversion applications with Gallium Nitride (GaN) high electron mobility transistors (HEMTs), the switching frequency is normally a few times higher than those using similar rated silicon devices. The increase in frequency can reduce the system's physical size and achieve a more compact design. However, the compactness of the module poses greater challenges on system's thermal design as the effective area for heat dissipation is reduced. A GaN based, buck converter module using copper core PCB with direct heatsink pad is designed and tested to demonstrate one possible methods of mitigating this issue. This module integrates input and output capacitors, four GaN transistors, four gate drivers and a coupled inductor to construct a two-phase buck converter that converts 12 to 5 V or 3.3 V. When operated at 500 kHz for a 12 to 5 V conversion with forced air cooling and additional heatsinks, this module can achieve a peak efficiency of 92.5% at 25 W of output power. It can deliver a maximum power of 176 W while maintaining the temperature of the GaN transistors to be below 90°C. At this maximum power, the power density is calculated to be 9.8 kW/L.