Objective To investigate the clinical characteristics of patients with gastric cancer and peritoneal metastasis (GCPM) and the associations of different treatment modalities with prognosis, thereby providing evidence for individualized clinical decision-making. Methods The clinical data of 173 patients with synchronous GCPM and no other distant metastases who were treated at The First Hospital of Jilin University between 2015 and 2024 were retrospectively analyzed. According to the treatments actually received, the patients were divided into six groups: chemotherapy alone; chemotherapy plus hyperthermic intraperitoneal chemotherapy (HIPEC); chemotherapy plus immunotherapy; chemotherapy plus immunotherapy and HIPEC; chemotherapy plus HIPEC and surgery; and chemotherapy plus HIPEC, surgery, and immunotherapy. Overall survival (OS) was estimated using the Kaplan-Meier method, and intergroup differences were compared using the Log-rank test. Pairwise comparisons were adjusted using the Holm method. Cox regression models were used to analyze prognostic factors. Because surgery was performed during the course of treatment, it was modeled as a time-dependent covariate. An extended Cox model was constructed according to the results of the proportional hazards assumption test, and a 6-month landmark sensitivity analysis was also performed. Follow-up ended on February 10, 2026. Results Among the 173 patients, 147 died during follow-up. The median follow-up time was 31.13 months, and the median OS was 13.57 months (95% CI 11.98-15.16 months).The median OS values in the six treatment groups were 4.10, 9.80, 12.57, 12.50, 16.40, and 18.60 months, respectively, with a statistically significant overall difference (P<0.001). After Holm adjustment, the chemotherapy plus HIPEC and surgery group differed significantly from the chemotherapy-alone group and showed a trend toward a survival advantage over the other three nonsurgical treatment groups. The chemotherapy plus HIPEC, surgery, and immunotherapy group differed significantly from all four nonsurgical treatment groups. Descriptive analysis showed that the median OS was longer in the surgery group than in the nonsurgery group (17.20 months vs.12.07 months, P<0.001). Univariable time-dependent Cox analysis showed that surgery was associated with a lower risk of death (HR=0.428, 95%CI 0.293-0.624, P<0.001).Multivariable extended Cox analysis indicated that the effect associated with surgery was time dependent (surgery-by-time interaction, P=0.027). In the 6-month landmark analysis, after adjustment for baseline factors, the difference in mortality risk between the surgery and nonsurgery groups was not statistically significant (HR=0.802, 95%CI 0.516-1.248, P=0.328). Univariable Cox analysis showed that both HIPEC (HR=0.457, 95%CI 0.325-0.643, P<0.001) and immunotherapy (HR=0.627, 95%CI 0.451-0.870, P=0.005) were associated with a lower risk of death. Multivariable extended Cox analysis showed that anemia, cT4b stage, and cN3b stage were associated with a higher risk of death. Conclusion OS differed among patients with GCPM who received different treatment modalities, and multimodal treatment regimens incorporating HIPEC and surgery were associated with favorable survival outcomes. In univariable analyses, HIPEC and immunotherapy were associated with a lower risk of death; the survival advantage associated with surgery was attenuated after treatment timing and baseline factors were taken into account. Anemia, cT4b stage, and cN3b stage were associated with poorer prognosis. These findings represent observational associations and should not be interpreted as evidence of independent causal effects of specific treatment modalities.