GaN-based transistors are a promising candidate for extreme environment conditions such as nuclear plants. In this paper, we present a recently selected industry standard Advance SPICE Model for GaN HEMTs (ASM-GaN), which can be used for simulating circuits which are designed for extreme environment conditions, where the device is exposed to different types of radiation, such as electron and proton radiations. GaN transistors exposed to different proton radiation dose levels exhibit different levels of degradation. The results presented in this paper demonstrate that the ASM-GaN model excellently captures variation in the electrical performance of the device under test for different radiation dose levels. The degradation in the large signal RF performance of power amplifier (PA) is predicted under different radiation dose levels.