GaN power transistors are drawing extensive research interest to increase power electronics systems efficiency and power density for electrical vehicles (EV). However, GaN devices suffer from current collapse, which results in shifted device static characteristics. In this paper, different measurement methods are proposed to investigate the influence of GaN transistor current collapse on device conduction and switching losses. By proposing a measurement circuit to accurately characterise GaN transistor dynamic RDSon, it is shown that device dynamic RDSon increases to 60\% bigger than its static RDSon value when it is operated at 1MHz converter. By proposing a modified double pulse test fixture, device switching transition can be characterised when it is unbiased. Therefore, it is shown that current collapse increases device switching losses 20\% bigger than unbiased condition. It is finally represented the influence of GaN device current collapse on device safe operation area in terms of operation current and frequency in EV-based power converters.