A gas detecting device configured to be attached to a surface includes a substrate, a semiconductor layer, a light-emitting component, a first electrode and a second electrode. The substrate includes a plurality of stacking layers stacked onto one another, and a material of the substrate includes cellulose nanofibrils (CNF). The substrate is formed by 3-D printing, such that a contact surface of the substrate is tightly attached to the surface. The semiconductor layer is formed on the substrate by 3-D printing. The light-emitting component is disposed on the substrate. The first electrode is coupled to the semiconductor layer and the light-emitting component. The second electrode is coupled to the semiconductor layer and a ground electrode. The first electrode and the second electrode are both disposed on the semiconductor layer and maintain a gap therebetween. A resistance of the semiconductor layer is changed according to a concentration of a designated gas.