Numerous High Pressure Gas Cables (HPGC) had been laid decades back by TenneT, the TSO of Netherlands and parts of Germany. Now, when TenneT decided to replace or increase the loadings on such cables, the question appears how would they behave to such temperature profiles and also the progress of their aging under such conditions. Only if a robust model is developed, can we understand which cable is closer to failure. Since all the cables cannot be replaced at once, prioritizing which ones are at the very near end and in the "red zone" of lifetime should be replaced first accordingly. But making a general model for all is not that simple, especially when there has to be a perfect balance between the academic research and also making it as realistic to the main scenario so that it can be applied on the field cables. Thus, the focus of the thesis is on observation of the physics-based processes taking place due to the effect of elevated temperatures on HPGC and the electro-thermal effect resulting in aging of insulation. The whole research was divided into three sub questions leading to solving the big problem eventually: 1) How does the insulation degrade when the field aged HPGC are elevated to higher temperatures, but lower than the maximum designed temperature? 2) Can diagnostic tests be developed or modified focusing on the material physics inside the insulation that can be performed on the cable alongside Tan Delta at 50 Hz and Partial Discharge Measurements, that is viable in field with least online time? 3) What parameters can be extracted from measurements to develop a robust model that determines the degradation of HPGC insulation with thermal and electrical aging? Initially, the cable loading profile was obtained from TenneT, cleaned using Machine Learning Algorithms and IEC Standard was applied to evaluate its operating temperature for last decade. Then, Fast, Short-Term Ramps as well as Long-Term Step AC Stress Tests were conducted on the samples to develop an aging model and evaluate the sensitivity of aging with elevated temperature. Once, an overall model has been developed, an electronic measurement box was designed to measure the Polarization and Depolarization currents (PDC) from unaged and aged samples. This was combined with a novel model developed using Frequency Dielectric Spectroscopy (FDS) for short duration online tests, and Partial Discharge (PD) Characterization. Using all the parameters obtained from the models fitted on the measurements of the experiments, trends were observed for these parameters which indicated the dielectric degradation with thermal aging. The conclusion clearly demonstrates how these indicators can be used during field tests to predict the progress of aging for these cables.