Symptoms of gastric cancer include indigestion and stomach discomfort or pain. These and other signs and symptoms may be caused by gastric cancer or by other conditions. In the early stages of gastric cancer, the following symptoms may occur: 1  Indigestion and stomach discomfort.2  A bloated feeling after eating. 3  Mild nausea. 4  Loss of appetite. 5  Heartburn.hese and other signs and symptoms may be caused by gastric cancer or by other conditions. In the early stages of gastric cancer, the following symptoms may occur: 1  Indigestion and stomach discomfort. 2  A bloated feeling after eating. 3  Mild nausea. 4  Loss of appetite. 5  Heartburn.