<b><i>Objective:</i></b> We built and externally validated a nomogram for predicting the overall survival (OS) probability of advanced gastric cancer patients receiving second-line treatment. <b><i>Methods:</i></b> The nomogram was developed on a set of 320 Italian patients and validated on two independent sets (295 Italian and 172 Korean patients). Putative prognostic variables were selected using a random forest model and included in the multivariable Cox model. The nomogram’s performance was evaluated by calibration plot and C index. <b><i>Results:</i></b> ECOG performance status, neutrophils to lymphocytes ratio, and peritoneal involvement were selected and included into the multivariable model. The C index was 0.72 (95% CI 0.68–0.75) in the development set, 0.69 (95% CI 0.65–0.73) in the Italian validation set, but only 0.57 (95% CI 0.52–0.62) in the Korean set. While Italian calibrations were quite good, the Korean one was poor. Regarding 6-month OS predictions, calibration was best in both Caucasian cohorts and worst the in Asian one. <b><i>Conclusions:</i></b> Our nomogram may be a useful tool to predict 3- or 6-month OS in Caucasian gastric cancer patients eligible for second-line therapy. Based on three easy-to-collect variables, the Gastric Life nomogram may help clinicians improve patient selection for second-line treatments and assist in clinical trial enrollment.