We present a valid and reproducible nomogram that combined the TNM stage as well as the Ki-67 index and carcinoembryonic antigen levels; the nomogram may be an indispensable tool to help predict individualized risks of death and help clinicians manage patients with gastric neuroendocrine carcinoma. To analyze the long-term outcomes of patients with grade 3 GNEC who underwent curative surgery and investigated whether the combination of carcinoembryonic antigen (CEA) levels and Ki-67 index can predict the prognosis of patients with gastric neuroendocrine carcinoma (GNEC) and constructed a nomogram to predict patient survival. In the training cohort, data were collected from 405 patients with GNEC after radical surgery at seven Chinese centers. A nomogram was constructed to predict long-term prognosis. Data for the validation cohort were collected from 305 patients. The 5-year overall survival (OS) was worse in the high CEA group than in the normal CEA group (40.5% vs. 55.2%, p = 0.013). The 5-year OS was significantly worse in the high Ki-67 index group than in the low Ki-67 index group (47.9% vs. 57.2%, p = 0.012). Accordingly, we divided the whole cohort into a KC(-) group (low Ki-67 index and normal CEA) and KC(+) group (high Ki-67 index and/or high CEA). The KC(+) group had a worse prognosis than the KC(-) group (64.6% vs. 46.8%, p < 0.001). KC(+) and the AJCC 8 stage were independent factors for OS. Then, we combined KC status and the AJCC 8 stage to establish a nomogram; the C-index and area under the curve (AUC) were higher for the nomogram than for the AJCC 8 stage (C-index: 0.660 vs. 0.635, p = 0.005; AUC: 0.700 vs. 0.675, p = 0.020). The calibration curve verified that the nomogram had a good predictive value, with similar findings in the validation groups. The nomogram based on KC status and the AJCC 8 stage predicted the prognosis of patients with GNEC well.