Is it common to get a diagnoses of stage four gastric/gi cancer without the primary source of the cancer confirmed? a tumor was confirmed on the liver

It does happen: Many tumors, especially without noticeable symptoms, are diagnosed following metastasis. Depending upon how the tumor appears under the microscope, a pathologist can often point out precisely where it came from, or, usually, a likely region. From there, it is a group effort, often involving pathologists, radiologists, and, in this case a gastroenterologist or general surgeon.

Not common.: It is not common but it does occur. Sometimes, the primary source is undetectable by imaging studies/endoscopies. Some think that the primary site was 'killed' by the immune system, while the cells that spread are different. They act differently, more aggressively.