With the increasing frequency requirements of 6G wireless communications extending beyond 110 GHz, the need for high-frequency, high-power amplifiers has become critical. While GaN-on-SiC HEMTs offer notable advantages over conventional silicon-based transistors, such as a wide bandgap and superior thermal conductivity, their application in high-frequency power amplifiers is limited by the relatively low gain of common-source configurations at these frequencies. The cascode topology, which integrates a common-source stage with a common-gate stage, provides a potential solution by improving impedance matching, isolation, and gain-bandwidth product. However, the adoption of cascode-based designs at D-band and beyond remains limited due to the lack of accurate high-frequency models for the common-gate configuration. This work presents the first comprehensive characterization and modeling of both common-source and common-gate GaN HEMTs from DC to 220 GHz. By overcoming existing model limitations, this research facilitates the development of high-power cascode amplifiers for next-generation wireless communication systems.