In this work, we studied transition metals (abbreviated as TM, including Ni, Pd, and Pt) embedded graphitic carbon nitride (g-C3N4) for adsorption of HCN gas using density functional theory (DFT) calculations. The results indicated that upon adsorption of HCN gas, the initial planar structures of the pure g-C3N4 and TM-embedded g-C3N4 systems (except Ni-embedded g-C3N4) are changing and the structures become corrugated. Furthermore, it was found that the d-orbitals of TM atoms hybridize with the p-orbitals of g-C3N4, leading to decrease of the band gap energy for the embedded systems when compared with the pure g-C3N4. In addition, our results of polarized band structure plots revealed that the asymmetry between the majority spin and minority spin branches of the Pt-embedded g-C3N4 give rise to spontaneous magnetization of this system with a net magnetic moment of about 1.35 mu B. Among the applied systems, the Pt-embedded g-C3N4 displayed substantially enhanced adsorption energy relative to the pure g-C3N4 (-0.297 eV), which is -1.98 eV. To the best of our knowledge the Pt-embedded g-C3N4 displayed the highest affinity for adsorption of HCN gas among the proposed adsorbents. Thus, the Pt-embedded g-C3N4 could be a low cost and an excellent candidate for sensing HCN gas and its removal from the atmosphere.