GaN High Electron Mobility Transistors (HEMTs) is very promising for high power switching and radiofrequency operation. However, the lack of reliability feedback is one of its major drawbacks. Trapping effect especially is one of the main performance limitations of such components. Many measurement techniques exist for trapping effects characterization, especially for short traps (µs to several ms). However for longer time constants, self-heating may distort the measurements. This paper presents an electrical and athermal transient measurement method which has been developed to study the trapping and detrapping time constants of such components. It allows the extraction of long transients without self-heating problems and is usable in long term electrical stress experiments. A simulation of this method with a simplified component's model and the measurements results are presented. With this technique, we investigated especially the long time constants (τ>20 ms) over a range of temperature from 10°C to 90°C. We observed three thermally activated trap signatures on GaN devices with our method.