Power density and efficiency are amongst the design features that are becoming extremely important in today's power electronics systems. Power switches made from Gallium Nitride (GaN) materials have recently become available commercially, allowing switching operations at considerably higher frequencies and hence making efficient and compact design possible for power converters. However, because of the unique structure of the power GaN switches which differ from conventional Si-based transistors, it is essential to realise their dynamic performance characteristics. In this paper, the dynamic response of an e-mode power GaN switching device made from Infineon (IGO60R070D1 650V/31A) is analysed using a dual pulse test (DPT) experiment. The switching characteristics of the device are investigated showing that special considerations are required to achieve reliable performance. In addition, a switching loss model is proposed based on the DPT results for the determination of both turn-off and turn-on switching losses at various operating conditions. The loss model can subsequently be used for design optimisation of the GaN power converters and their cooling systems for various applications such as automotive and photovoltaic systems.