This paper presents a comprehensive study on the relation between die-attach and thermal/electrical parameters of GaN RF devices. This correlation is investigated through Multiphysics simulations and experimental data. Particularly, thermal analysis is performed by means of Quantum Focus Instrument (QFI) Infrascope able to detect the surface temperature of the device. Then, 3-D finite element method thermal simulations are performed to support the observed heat distribution. A strong association between drain current drift and temperature escalation is demonstrated by comparing two devices with significantly different die-attaches. Particularly, we observe an increase in the drain current with increasing self-heating effects, conversely to what generally expected for thermal derating. However, this correlation is then explained thanks to the analysis of threshold voltage shift with temperature that supports the experimental evidence.