The purpose of this Phase 2/3 study is to evaluate efficacy and safety of the combination of selinexor and R-GDP (SR-GDP) in patients with RR DLBCL who are not intended to receive hematopoetic stem cell transplantation (HSCT) or chimeric antigen receptor T cell (CAR-T) therapy. This study consists of 3 arms each in Phase 2 and 3. Phase 2 portion of the study will assess the two doses of selinexor (40 milligram \[mg\] or 60 mg) in combination with R-GDP, for up to 6 cycles (21-day per cycle), followed by 60 mg selinexor single agent continuous therapy for those who have reached a partial or complete response. Phase 3 portion of the study will evaluate the selected dose of SR-GDP (identified in Phase 2) versus standard R-GDP + matching placebo, for up to 6 cycles (21-day per cycle), followed by placebo or 60 mg selinexor single agent continuous therapy for those who have reached partial or complete response.