We have engineered and commissioned the state-of-the-art 400/220/66 kV Gas Insulated Substation (GIS) at Wangtoo, the first high-voltage substation in Himachal Pradesh. Constructed at an altitude of 1700 m above MSL, the GIS will now be a major power pooling station to evacuate power from the upper Sutlej basin. Our scope involved design, construction, testing and commissioning of the substation along with allied works across a very challenging terrain comprised 10 different gradients under extreme climatic conditions.