This paper focuses on the modelling of a nano -GNRFET for gas sensing using Sentaurus TCAD. Gas sensing is done by diffusing a gas over the surface of the Graphene Nano Ribbon (GNR) layer. Doped GNR is placed over the gate separated by a thin layer of insulator. A V ASP structure of a zigzag GNR (ZGNR) doped separately with Gallium Phosphorus as dopants has been first modelled for observing their structure and atomic spacings in free space. Results of the TCAD analysis indicate that the adsorbance of NzO by the doped GNR layer is reflected by a sizeable change in the N-well potential, making these structures promising candidates for nano scale sensing of NzO.The width of the Graphene nano ribbon is 90nm x 500nm. 28nm fabrication technology is used for modelling the GNRFET. Doped GNR generally has good conductivity which yields in better gas sensing.