In this work, a novel light-modulated bifunctional gas sensor based on Au nanoparticles-modified 2D InSe nanosheets was demonstrated. The prepared sensor displayed a reversible and extremely high response for recognition of nitrogen dioxide (NO) under visible-light illumination. The sensitivity (1192%) was about 10 times higher than that under dark condition, and the limit of detection (LOD) was 0.17 ppb. In contrast, when sensing ammonia (NH), higher sensitivity and selectivity were obtained in darkness rather than in light, with sensitivity and LOD of 11% and 0.2 ppm. Furthermore, the sensor possesses decent stability, repeatability, and anti-interference ability. The tunable sensing behavior with light modulation has been clearly studied with the help of density functional theory. A new principle called "carrier storage box" of Au nanoparticles was proposed to explain the change in surface state of InSe under light modulation. Finally, the prepared sensor has been successfully applied to construct a fully integrated wearable device to measure NH and NO in ambient environment. In all, this work provides a highly competitive gas detection method and paves the way for designing 2D materials-based optoelectronic devices with tunable and multifunctional features.