A pathway frequently altered in cancer is glutaminolysis where glutaminase (GA) catalyses the main step: the deamidation of glutamine to form glutamate and ammonium. There are two types of GA isozymes, named GLS and GLS2, which differ considerably in their expression patterns and can even perform opposing roles in cancer. GLS correlates with tumor growth and proliferation, while GLS2 may function as a context-dependent tumor suppressor. However, both isoenzymes have been described as essential molecules handling oxidant stress because of their involvement in glutathione production. We have reviewed the literature to highlight the critical roles of GLS and GLS2 to restrain ROS and regulate both cellular signaling and metabolic stress due to their function as indirect antioxidant enzymes and also by modulating both reductive carboxylation and ferroptosis. Blocking GA activity appears to be a potential strategy to dual activate ferroptosis and inhibit cancer cell growth in a ROS-mediated mechanism.