Inexpensive and compact gas sensing systems are indispensable for accurate air quality monitoring with broad spatial coverage. We report an InGaZnO thin-film transistor (TFT) based integrated NO2 gas detection system that yields a digital output proportional to the NO2 concentration. This is the first report on the integrated TFT based detectors where gas concentration is quantified and provided simultaneous digital output without additional signal conditioning or analog to digital conversion circuits. It is a low power consuming system with consumption in the order of a few microwatts. It constitutes of highly sensitive and selective fabricated InGaZnO TFT gas sensor operable at room temperature. The response of the TFT in the presence of the NO2 as a sensor is evaluated and it is reproducible with the measured limit of detection as 100 ppb. Such standalone thin-film electronic systems that function as a gas-sensitive analog to digital converters pave the way to the compact and inexpensive gas sensing systems.