Wide band gap semiconductors are expected to be the future technology for power semiconductors that can enable the reduction of power consumption in many applications. But, in order to use these materials, it is necessary to study their reliability. Regarding its possible use in radioactive environments, it is essential to know the response of HEMT structures based on GaN under radiation. This paper is focused on the study of the effects of gamma radiation on dynamic performance of commercial GaN HEMTs. A test campaign was performed to detect variations on dynamic on-resistance induced by irradiation, due to the relevance of dynamic resistance behavior in power switching converters. This study demonstrates an increase of the dynamic resistance in MISHEMTs structures when they are under gamma radiation together with a voltage stress in the drain region. This increase takes place especially after a hard- switching transition due to the hot-electron effect that takes place during the switching events and can not only lead to an increase of the power losses, but also a reduction in the life time of the device due to the permanent degradation that could be induced by the hot-electron effect.