The adsorption behavior of NO gas on the pristine graphitic carbon nitride (gCN) and transition metal atoms (TM).embedded gCN systems (TM = Fe, Ru, and Os atoms) was investigated with density functional theory in solid state. The adsorption energy results revealed that the embedding of TM atoms can significantly improve the adsorption properties of NO gas on the gCN systems with high adsorption energy of -3.14 eV for Os-embedded gCN, compared with that on the pristine gCN (-0.54 eV). Our results indicated that the adsorption energy of NO gas on the Os-embedded system is much larger than those reported for the other adsorbents. Although NO gas was physisorbed on the pristine gCN, this gas was strongly chemisorbed over the TM-embedded systems. Furthermore, it was observed that the electronic and magnetic characteristics of gCN considerably modulated by embedding of transition metal atoms. The results of magnetic calculations displayed that Os-embedded gCN exhibited magnetic properties with magnetic moment of 0.77 mu(B), whereas Fe and Ru-embedded gCN systems are non-magnetic. Furthermore, it was found that the conductivity of TM-embedded gCN systems was more than that of the pristine system, due to the induced TM atoms impurity states in the band gap energy. In addition, the results displayed electron donation from d-orbitals of TM atoms to pi* orbital of NO gas for all of the TM-embedded gCN systems. Therefore, the results displayed that among all of the TM-embedded gCN systems, the Os-embedded gCN has the best adsorption characteristics for sensing and removing of NO gas from the environment.