Objectives: The objective of this study was to compare the usefulness of two inflammation-based prognostic scores, neutrophil to lymphocyte ratio (NLR) and modified Glasgow Prognostic Score (mGPS), assessed at diagnosis in stage IV advanced gastric cancer (AGC). Methods: We retrospectively reviewed the medical records of 104 patients with newly diagnosed metastatic AGC treated with palliative chemotherapy. Results: In the univariate analysis, the following variables were associated with shorter overall survival (OS): poor or undifferentiated histology (p = 0.013), more than 1 metastasis (p = 0.004), the presence of lymph node metastasis (p = 0.003), the presence of bone metastasis (p = 0.019), a lower albumin level (p < 0.001), elevated C-reactive protein (p < 0.001), a high absolute neutrophil count (p = 0.016), NLR ≥3 (p < 0.001) and higher mGPS (p < 0.001 and p = 0.007, respectively). In the multivariate analysis, high NLR and mGPS were independent prognostic factors for shorter OS (p = 0.037, p < 0.001 and p = 0.010, respectively), along with lymph node metastasis (p = 0.005) and histological subtype (p = 0.048). Conclusion: This study suggests that the inflammatory markers, NLR and mGPS, are independent prognostic factors for OS in patients with unresectable AGC treated with palliative chemotherapy.