3-Level Flying Capacitor Topology enables the operation of 650 V GaN Transistors for automotive DCDC-converters at 800 V DC-Link voltage. The very fast switching of GaN devices recommends an ultra low inductive design of the commutation circuits. In this contribution, this is realized with GaN bare die transistors on a novel 5-level AMB substrate with internal mass layer, which is additionally used for EMI shielding and thermal spreading. To characterize the switching behaviour, double pulse tests for the commutation paths of the 4 switches are performed. Additional measurements at a 3-level flying capacitor half bridge module with fast IGBTs are used for comparison. The results show a proper switching behaviour of the GaN module, and oscillations can be avoided by a moderate increasing of gate resistance. The IGBT module can be operated also at fast switching speed when driven with a gate resistance far below recommendation. For the GaN AMB module, additional measurements of the parasitic commutation loop inductances show proper values in the lower single-digit nanohenry range, whereas first tests concerning switching energies do not provide a clear picture.