Highly stretchable, sensitive and room-temperature nitrogen dioxide (NO2) sensors are fabricated by exploiting intrinsically stretchable, transparent and ion-conducting hydrogels and active metals as the novel transducing materials and electrodes, respectively. The NO2 sensor exhibits high sensitivity (60.02% ppm-1), ultralow theoretical limit of detection (6.8 ppb), excellent selectivity, linearity and reversibility at room temperature. Notably, the sensitivity can be maintained even under 50% tensile strain. For the first time, it's found that the metal electrodes significantly impact the sensing performance. Specifically, the sensitivity is boosted from 31.18 to 60.02% ppm-1 by replacing the anodic silver with copper-tin alloy. Importantly, by applying specially designed sensing tests, and microscopic and composition analyses, we have obtained the inherent NO2 sensing mechanism: the anodic metal tends to be oxidized and the NO2 molecules tend to react in the cathode-gel interface. The introduction of glycerol converts the hydrogel into the organohydrogel with remarkably enhanced anti-drying and anti-freezing capacities and toughness, which effectively improved the long-time stability of the sensors. Importantly, we execute sound/light alarms and a wireless smartphone alarm by utilizing a designed circuit board and applet. This work gives an incisive investigation for the preparation, performance improvement, mechanism and application of hydrogel-based NO2 sensors, promoting the evolution of hydrogel ionotronics.