Gastric-Esophageal Cancer

The goal of this clinical trial is to assess if circulating tumor DNA, as assessed by Signatera tumor-informed MRD assay, can guide de-escalation of adjuvant therapy in patients with resectable Stage II-IVa Gastric/Gastric-Esophageal Cancer. The main question it aims to answer is: • To demonstrate if 2-year disease-free survival (DFS) among participants with resectable clinical Stage II-IVa gastric/gastro-esophageal cancer who received neoadjuvant D-FLOT and are ctDNA negative after curative intent R0 surgery receiving adjuvant durvalumab monotherapy is non-inferior to standard of care D-FLOT. Researchers will compare post-operative ctDNA negative participants who are receiving adjuvant durvalumab monotherapy to those who receive standard of care (D-FLOT) to see if they have similar outcomes. Participants will be asked to: * Receive serial ctDNA testing * Visit their study doctor per their standard of care visits about every 3 months for first 2 years and then every 6 months for an additional 3 years * Answer 5 questionnaires about their well-being