In the industry era, nanostructures-based gas sensors have been more striking for environmental monitoring. In this research, we investigated the properties and the adsorption ability of boron carbon nitride (BCN) and in-plane graphene-boron nitride (G_BN) nanosheets for toxic (F₂ and O₃) gas molecules. The geometrical, electronic, and optical properties of the selected structures are analyzed using the density functional theory. The negative value of cohesive energy signifies that both BCN and G_BN are energetically stable. Both structures show strong attractive energy for the selected gas molecules with recovery times of 0.266 ps - 5.482 ns. A significant change decrease in the band gap of the adsorbents occurred after gas adsorption. All the adsorbents show a high absorption coefficient of over 10⁴ cm^-1 in the visible wavelength range. A slight variation in the optical properties is observed after gas adsorption. Both the adsorbents demonstrate significant sensitivity toward F₂ and O₃ gases.