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First-principles calculations based on spin-polarised density functional theory have been used here to investigate the structural, electronic and magnetic properties of tape-heme monolayer (THM) with gas molecules (H2O, N2, HCN, NH3, NO2 and SO2) adsorption. Our results suggest that H2O and NH3 are physisorbed on THM while N2, HCN, NO2 and SO2 are adsorbed on THM by chemisorption. The toxic HCN, NH3, NO2 and SO2 can be effectively captured by THM according to their adsorption energy (> 0.50 eV). The adsorption of NO2 (or SO2) can partly (or completely) decrease the spin moment of THM and induce the transition from the metallic property into half-metallic (or semiconductor) behaviour. The magnetism of the NO2 adsorption system is sensitive to tensile strain while that of the SO2 adsorption system is invariably non-spin polarised regardless of the compressive or tensile strain. The electronic structure of the NO2 and SO2 adsorption system can be effectively modulated by external biaxial strain. Our findings reveal that THM could effectively detect toxic NO2 and SO2 in an environment with high sensitivity and selectivity and reactivity, indicating that it possesses a potential application in the field of heme-based gas sensors. [ABSTRACT FROM AUTHOR]