The focus of this paper is to provide a better understanding on the role of the different layers as well as the gate length in determining thermal time constants in GaN High Electron Mobility Transistors (HEMTs). The paper is divided in two parts. In the first one, a theoretical approach is proposed where a comparison between numerical Finite Element Analysis (FEA) coupled to Model Order Reduction technique based on Ritz vector method and analytical approach is performed. It has been shown that real physical thermal time constants obtained from the analytical solution of the heat equation of simple realistic structures which represent GaN HEMTs can be obtained through a coupled approach between a numerical solution and a model order reduction technique. The second part explores a measurement approach to identify the transient time constant behavior of GaN HEMT devices. This measurement technique is based on a transient thermoreflectance imaging.