There has been no report on whether the patients with gastrointestinal cancer who are also inactive hepatitis B carriers should receive prophylactic use or preemptive use of an anti-viral drug entecavir during anti-tumor therapy. This open, multicentre, phase 3, randomized controlled clinical trial aims to compare the impact of the prophylactic use or preemptive use of an anti-viral drug entecavir on the outcomes of patients with gastrointestinal cancer who are also inactive hepatitis B carriers during chemotherapy or immunotherapy and the subsequent follow-ups, including two cohorts of chemotherapy and immunotherapy.