Commercial gallium nitride (GaN) high-electron-mobility transistors used for power electronics applications show superior performance compared to silicon (Si)-based transistors. Combined with an increased radiation hardening properties, they are key candidates for high-performance power systems in a harsh environment, such as space. However, for this purpose, it is key to know the potential failure mechanisms (FMs) of the devices in depth. Here, we demonstrate how the repeated thermomechanical stress in a power cycling (PC) test within specified operating conditions destroys the GaN device. Based on leakage current localization analysis, we identify an FM with a yet unknown root cause. Utilizing emission microscopy, focused ion beam cutting, and scanning electron microscope techniques, it is revealed that multilayer cracks of a GaN die are triggered by a commercial leading package structure, which shows excellent capability under frequent thermomechanical stress. Through multiphysics simulations, it is shown that the structural factors that lie behind the strong performing component properties inside the package ultimately are directly related to the failure pattern. This article is accompanied by a video demonstrating dynamic thermal distribution difference between thermography measured in a practical experiment and a multiphysics simulation result during a single PC of a PC test. This article is accompanied by a supplementary figures file demonstrating test environment, preparation process of specimens, and reverse engineering results for the simulation model.