Gas Insulated Substations (GIS) are gaining popularity over Air Insulated Substations (AIS) due to less space requirements and increased reliability. However, the switching surges produced with Disconnector operations are causing electromagnetic disturbances in the control circuits of Gas insulated substations. In this paper, Electro Magnetic Transient Program software is used for estimating Very Fast Transient Over Voltages at various locations in a 420 kV Gas-Insulated Substation. The peak VFTO is estimated for different trapped charge values on the bus section. The maximum peak of VFTO is observed with a trapped charge voltage of −1 pu is 2.09 pu. From the Frequency response analysis of the VFT overvoltage waveforms, it has been found that the dominant frequency elements lie in the range of 1 MHz–70 MHz.