Background <p>Gastric cancer is a major cause of cancer-related mortality, and reliable postoperative risk prediction is essential in critically ill patients. The C-reactive protein–albumin–lymphocyte (CALLY) index reflects inflammatory and nutritional status, while lactate indicates metabolic stress and tissue hypoperfusion. This study aimed to develop a lactate-based CALLY (CALLY-I) score and evaluate its prognostic value in postoperative gastric cancer patients admitted to the intensive care unit (ICU).</p> Methods <p>This retrospective study included 186 patients who underwent gastric cancer surgery and required postoperative ICU care between January 2022 and January 2025. Demographic characteristics, laboratory parameters, APACHE II and SAPS II scores, and derived indices (CALLY and CALLY-I) were analyzed. Predictors of 30- and 90-day mortality were assessed using Receiver Operating Characteristic (ROC) analyses, and significant variables were included in multivariate logistic regression.</p> Results <p>The 30-day and 90-day mortality rates were 18.2% and 26.7%, respectively. Lactate demonstrated the strongest predictive ability for both 30-day (AUC = 0.865) and 90-day (AUC = 0.806) mortality. The CALLY-I score showed superior prognostic performance compared with the classical CALLY index and was an independent predictor of 30-day mortality (OR = 0.47, p = 0.032). APACHE II, albumin, and BUN were also independently associated with mortality, while the classical CALLY index did not show independent significance.</p> Conclusion <p>The lactate-based CALLY-I score enhances prognostic accuracy by combining metabolic, inflammatory, and nutritional markers. CALLY-I may serve as a practical tool for postoperative risk stratification in gastric cancer patients requiring ICU monitoring. Prospective multicenter studies are needed for further validation.</p>