If left untreated, will gastrointestinal bleeding harm you?

Depends: This strongly depends on the cause of the bleeding. Any undiagnosed GI bleeding warrants further evaluation. There are several potentially minor and self limited causes for bleeding, but there are also some very concerning or dangerous causes like cancers or ulcers. If you don't have a diagnosis for bleeding, then see your doctor.

It may: It depends on the cause and the severity. Bleeding may be a symptom of cancer which if untreated would lead to death. Severe bleeding causes anemia which can lead to death. All GI bleeding should be evaluated by at least a doctor visit.