A Study Evaluating Efficacy and Safety of Mosunetuzumab in Combination With Polatuzumab Vedotin Compared to Rituximab in Combination With Gemcitabine Plus Oxaliplatin in Participants With Relapsed or Refractory Aggressive B-Cell Non-Hodgkin's Lymphoma

Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2 * CD20+ aggressive lymphoma as determined by the local hemopathology laboratory from the following diagnoses by 2016 World Health Organization classification of lymphoid neoplasms: DLBCL, not otherwise specified (NOS); high-grade B-cell lymphoma (NOS or double/triple hit); transformed follicular lymphoma; follicular lymphoma Grade 3b * Have disease relapsed or refractory to at least one prior systemic therapy for aggressive non-Hodgkin's lymphoma (aNHL) * Participants who have received only one prior line of therapy must be ineligible for autologous stem cell transplant (ASCT) * Measurable disease * Adequate hepatic, hematologic, and renal function * Estimated creatinine clearance (CrCl) ≥ 30 mL/min by Cockroft-Gault method or other institutional standard methods * Negative HIV test at screening. Participants with a positive HIV test at screening are eligible provided that, prior to enrollment, they are stable on anti-retroviral therapy for at least 4 weeks, have a CD4 count of at least 200 microliters, have an undetectable viral load, and have not had a history of opportunistic infection attributable to AIDS within the last 12 months Exclusion Criteria: * Pregnant or breast feeding, or intending to become pregnant during the study or within 3 months after the final dose of mosunetuzumab, 9 months after the final dose of polatuzumab vedotin, 12 months after the final dose of ri…