Using first-principles calculations coupled with thermodynamic analysis, we have explored the sensing behavior of the two-dimensional MoSi2N4 (2D MSN) monolayer towards selected hazardous gas molecules, such as CO, CO2, NO, NO2, SO2, H2S, NH3, CH4, methanol (CH3OH), ethanol (C2H5OH), and acetone (C3H,6O). We find that the incident molecules bind weakly (0.13 to 0.36 eV) on pristine monolayer, however the adsorption energies improve significantly (0.38 to 0.86 eV) on the charged MSN monolayer because of the enhanced electrostatic interaction caused by appreciable charge transfers (0.36e to 0.67e). In particular, negatively charged MSN monolayers exhibit a substantial improvement in the adsorption of H2S and NH3, while both negatively and positively charged MSN monolayers showed enhanced adsorption towards NO and NO2. Appropriate adsorption energies are coupled with quantifiable changes in the electronic properties, and variation in the work function of MSN monolayer, which authenticate its potential as efficient nanosensor. Application of thermodynamic analysis further validate the reversible sensing characteristics of MSN monolayer at ambient conditions. Our findings show that gated MSN monolayers can serve as sensitive and selective nanosensors towards common pollutants.