Gas-involving cancer theranostics have attracted considerable attention in recent years due to their high therapeutic efficacy and biosafety. We have reviewed the recent significant advances in the development of stimuli-responsive gas releasing molecules (GRMs) and gas nanogenerators for cancer bio-imaging, targeted and controlled gas therapy, and gas-sensitized synergistic therapy. We have focused on gases with known anti-cancer effects, such as oxygen (O2), carbon monoxide (CO), nitric oxide (NO), hydrogen sulfide (H2S), hydrogen (H2), sulfur dioxide (SO2), carbon dioxide (CO2) and heavy gases that act via the gas-generating process. The GRMs and gas nanogenerators for each gas have been described in terms of the stimulation method, followed by their applications in ultrasound and multimodal imaging, and finally their primary and synergistic actions with other cancer therapeutic modalities. The current challenges and future possibilities of gas therapy and imaging vis clinical translation have also been discussed.