•Single Mo atom embedded at the six-fold cavity with high stability.•Mo decorated g-C3N4 monolayer is promising adsorbent for HCN, H2S, and SO2 and NO.•HCN, SO2, and NO act as acceptors and H2S acts as donor.•The adsorption of HCN, SO2, H2S makes the system exhibit semimetal properties.•The magnetism be regulated by the gas molecule adsorption. In this paper, we explored the adsorption structure, electronic states, and charge transfer of toxic gas molecules, including HCN, SO2, H2S and NO, on pristine and Mo atom embedded graphitic carbon nitride (g-C3N4) by first-principles calculations. Our results show that single Mo atom prefers to be embedded at the six-fold cavity with high stability. The adsorption of those gas molecules on pristine g-C3N4 monolayer exhibits weak interaction, while Mo atom doping can significantly enhance their adsorption energy. Partial density of states analysis illustrate strong hybridization among Mo-4d, gas molecule and g-C3N4. Charge transfer analysis indicates that HCN, SO2, and NO act as acceptors, while H2S acts as donor. The magnetism of g-C3N4/Mo also undergoes great change after the gas molecules adsorption. The adsorption of HCN, SO2, H2S makes the system exhibit semimetal properties, while the adsorption of NO produces much smaller magnetic moment. Therefore, the magnetism of the Mo/g-C3N4 can be modified by the gas molecule adsorption. Our calculations demonstrate that Mo decorated g-C3N4 monolayer can be a promising adsorbent for gas molecule and the variation of magnetic moment through gas adsorption can also be utilized.