Gastric cancer has a poor prognosis despite available treatments. Inclusive studies are still needed with real-life data. Our research retrospectively evaluated the efficacy of trastuzumab and potential risk factors on survival in patients with HER2-positive metastatic gastric cancer who received trastuzumab-based chemotherapy as first-line therapy. In total, 138 patients were included in this study. Clinical response to trastuzumab-based chemotherapy was obtained in 79% of the patients. We also divided the patients who had a clinical response into two groups according to whether they received maintenance therapy. In the present study, trastuzumab administration had compatible survival outcomes with recent studies. Continuation of trastuzumab maintenance treatment provided a survival advantage in patients with clinical response. We suppose that maintenance trastuzumab may be recommended in patients with clinical responses to the first-line treatment. Furthermore, Eastern Cooperative Oncology Group Performance Status 2, grade 3 and having more than four metastatic lesions were determined as risk factors for death. Therefore, although we recommend maintenance of trastuzumab in patients with clinical response, those with identified risk factors may not benefit from treatment.