Oxidative stress (OS) occurs when there is an imbalance between oxidants and antioxidants, leading to disruptions in cellular signaling and causing damage to molecules. Glutathione S-transferase (GST) enzymes are crucial for maintaining redox balance by facilitating glutathione conjugation. Increased oxidative damage has been noted during the COVID-19 pandemic, and its persistence may be linked to the onset of long COVID. This systematic review aimed to assess the relationship between gene polymorphisms and the prognosis of COVID-19, including long COVID. Adhering to the PRISMA guidelines, a thorough search was carried out in MEDLINE, CENTRAL, PubMed, and EMBASE for studies published from September 2020 to February 2025. Out of an initial selection of 462 articles, ten studies (four concerning COVID-19 severity and six related to long COVID) satisfied the inclusion criteria. The findings regarding polymorphisms were not consistent, especially concerning the and isoforms. Nevertheless, evidence indicates a sustained state of oxidative stress in patients with long COVID. The majority of research has focused on cytosolic GSTs, while the functions of microsomal and mitochondrial GST families remain largely unexplored. These findings suggest that further research into the various subfamilies and their genetic variants is necessary to enhance our understanding of their impact on COVID-19 severity and the pathophysiology of long COVID.