Objective: Metastatic gastric cancer (mGC) is an incurable disease and a leading cause of cancer-related deaths worldwide. The prognostic significance of systemic inflammation and nutritional scores in patients with mGC has been investigated; however, optimal biomarkers for prognosis need to be identified. Material and Methods: This single-center retrospective study included patients with synchronous or metachronous mGC. We evaluated the associations between overall survival (OS) and Eastern Cooperative Oncology Group performance status (ECOG PS), serum albumin level, neutrophil-to-lymphocyte ratio, platelet-to-lymphocyte ratio, systemic immune-inflammation index, C-reactive protein-to-albumin ratio (CAR), prognostic nutritional index, modified Glasgow prognostic score (mGPS), and inflammatory burden index. Results: In total, 203 patients were included, with 144 (71%) males and 59 (29%) females. The median age was 59 years (range: 21-82). The median follow-up time was 13.9 months (range: 2.7-114.9 months). Univariate analysis revealed that the ECOG PS (p=0.001), body mass index (BMI) (p=0.006), serum albumin level (p=0.002), CAR (p=0.013), and mGPS (p<0.001) were significant prognostic factors for OS. In the multivariate analysis, ECOG PS≥1 vs. 0 [Hazard Ratio (HR: 1.5, 95% CI: 1.07-2.48; p=0.018) Confidence Interval], BMI<23.20 kg/m2 vs. ≥23.20 kg/m2 (HR: 0.70, 95% CI: 0.53-0.98; p=0.037) and mGPS 2 vs. 0-1 (HR: 1.3, 95% CI: 1.1-1.7; p=0.001) were independent predictors of poorer OS. Conclusion: Our findings suggested that pretreatment BMI and the mGPS may be significant prognostic biomarkers for predicting OS in patients with mGC. A low BMI and high mGPS are associated with poor survival outcomes.