Fecal Microbiota Transplantation (FMT) in Patients With Advanced Gastric Cancer

This trial tests fecal microbiota transplantation (FMT) — transferring healthy gut bacteria from a donor into a patient — as a potential way to improve outcomes in people with advanced stomach cancer. Researchers believe that the gut microbiome may influence how well immunotherapy works. **You may be eligible if...** - You are 18 or older - You have been diagnosed with advanced, unresectable, or metastatic stomach or gastroesophageal junction cancer, confirmed by biopsy - At least one tumor can be measured on scans - Your performance status is good (ECOG 0 or 1) - Your life expectancy is at least 3 months **You may NOT be eligible if...** - You have a condition that makes colonoscopy unsafe (such as toxic megacolon or inflammatory bowel disease) - You have had a colectomy - You are already in another clinical trial Talk to your doctor to see if this trial is right for you.