A high-density natural cooling half-bridge power module for a top-cooled 650-V GaN HEMTs is proposed in this paper. Compared with Si and SiC devices, GaN HEMTs have advantages, such as low on-resistance, low drive loss, low switching loss and small size. However, they also have challenges such as large switching oscillations and small heat dissipation area. This paper proposes a PCB layout scheme with low parasitic impedance and bipolar gate drive with +6 V/−3 V. It keeps GaN devices completely on one side of the PCB while making the parasitic inductance of the power loop within 1 nH. Hence, the installation of heat sinks becomes easier. This paper verifies the above design in a CCM boost converter that is widely used in photovoltaic and energy storage systems. A loss evaluation based on the SPICE model of GaN device GS66516T is given. Finally, a 2.5 kW experimental prototype with the proposed power module is manufactured. It was tested to achieve peak efficiency of 99.14% and full-load efficiency of 99.09%. The experimental results show that the temperature rise of GaN HEMTs is limited within 50 °C in a windless environment, which effectively proves the rationality of the proposed power module design and loss evaluation method.