This research explores insulation coordination in a 230 kV GIS using advanced power system simulation tools. Unlike traditional Medium Voltage (MV) systems with conservative insulation designs, High Voltage GIS installations require precise assessment of insulation performance under various transient conditions. Lightning and switching surges are critical factors influencing GIS insulation integrity. The study simulates over-voltage events to optimize insulation levels and strategically position protective devices. A significant aspect of this approach is the use of MOSA, which offer rapid response and high energy absorption during transient events. MOSAs, primarily made of zinc oxide (ZnO), surpass older spark gap-based Arresters due to their nonlinear voltage-current characteristics and self-restoring capability. This research aims to enhance the reliability and longevity of GIS installations by ensuring effective insulation coordination and protection against over-voltage threats.