The threshold voltage (<i>V</i>_T) shift and anomalous hump characteristics caused by external gamma-rays (γ-rays) are problematic in electronic devices, especially in aerospace applications. To overcome those issues, we investigated the degradation mechanism and radiation immunity in ultrathin amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) with a sub-10 nm film, utilizing X-ray photoelectron spectroscopy (XPS) analysis and subgap density-of-states (DOS) characterization. Furthermore, energy-efficient electrothermal annealing (ETA) with a power-optimized electrical pulse signal is adopted to recover inevitable device damage triggered by continuous irradiation, resulting in enhancement of subthreshold slope (SS), elimination of the abnormal hump phenomenon, and confirmed heat spreading through thermal simulation. Importantly, our approach provides evidence of a specific thickness configuration exhibiting superior γ-ray immunity. This study can pave the way for device design guidelines for oxide-based TFTs, which can be applied to future highly reliable electronic devices under harsh environments.