Chronic Graft-versus-Host Disease (cGVHD) is a common late complication following allogeneic hematopoietic stem cell transplantation and a leading non-relapse cause of death. It is often treatment-refractory, significantly affecting patients' quality of life and prognosis. This study will evaluate the feasibility, safety, and tolerability of gecacitinib, a novel JAK and ACVR1 inhibitor, in 24 patients with moderate-to-severe cGVHD who have undergone two or more prior therapies. Participants will receive gecacitinib hydrochloride tablets for at least 24 weeks. Patients demonstrating disease stability, as assessed by the investigator, may continue treatment with the study drug until week 60, unless intolerability, disease progression, or initiation of new systemic therapy, whichever occurs first.