Gas insulated switchgears (GIS) have the best insulation performance ensuring least failure incidents and most reliable operation, which justify the higher installation costs. Nevertheless, this popular switchgear also needs partial discharge measurement for insulation diagnosis, as it does detailed modeling and analysis of the insulation is needed. This paper presents a novel computational model of the complex pre-breakdown discharge phenomena that occur due to insulation imperfection in GIS. The paper aims to provide quantifiable parameters modeling equivalent to industrial measured parameters during commissioning of GIS. The proposed model is based on Poisson's equation along with hydrodynamics equations which are utilized to investigate the influence of defect dimensions, electrical pulse features on pre-breakdown incidents and on the alteration of the charge density, electric stress, and temperature distributions around the insulation imperfection.