Visible-light activation is highly desirable for gas sensors due to its energy-efficient operation and broad accessibility. Photocatalysis offers a promising strategy for visible-light activation; however, a limited understanding of the band engineering-mediated activation process restricts the rational design of photocatalysts for gas sensors. In this work, we systematically investigate the impact of band tuning in photocatalysts on the nitrogen dioxide (NO<sub>2</sub>) sensing performance of In<sub>2</sub>O<sub>3</sub>-based sensors, employing graphene quantum dots (GQDs) as photosensitizers. By controlling the sp<sup>2</sup> carbon core size in GQDs, the bandgaps are tuned from 3.3 to 1.9 eV, enabling precise band engineering. It modulates the carrier transfer dynamics between GQDs and In<sub>2</sub>O<sub>3</sub> layers, while surface functional groups of GQDs facilitate gas adsorption through their catalytic effects. By integrating sensitization effects, 7 nm GQDs optimize the photocarrier efficiency under visible light (blue light), leading to enhanced NO<sub>2</sub> sensing performance in the GQD-decorated In<sub>2</sub>O<sub>3</sub> system (<i>R</i><sub>g</sub>/<i>R</i><sub>a</sub> = 97.1 toward 1 ppm) with a fast response/recovery time (<i>T</i><sub>90</sub>/<i>T</i><sub>10</sub> = 136/100 s). The bandgap tuning of GQDs highlights the critical role of band engineering in light-assisted gas sensing, enabling the photocatalyst-based sensor system construction for visible-light activation.