The voltage rating of the commercial Gallium Ni-tride (GaN) power devices are limited to 600/650 V due to the lateral structure. Stacking the low-voltage rating devices is a straightforward approach to block higher dc-link voltage. However, the unbalanced voltage sharing can occur due to the discrepancies in the gate driving loops, the device parameter tolerance and the device-to-ground displacement currents for the series-connected devices in the stack. In this paper, a novel closed-loop current source gate driver is proposed, which addresses the voltage imbalance issue of series-connected GaN HEMTs for both hard switching and soft switching scenarios. The proposed current source gate driver controls the device switching timing and the dv/dt with fine accuracy by directly regulating the device gate current. Without the employment of the lossy snubber circuit or the external Miller capacitor, the switching energy and the switching speed are almost not compromised for each individual device. Meanwhile, the closed-loop strategy improves the adaptivity to different operating conditions. A series-connected GaN-based multiple pulse tester is built to validate the proposed current source gate driver and the voltage balancing strategies in different switching scenarios.