This is a Phase 1/2 study of GB261 in participants with relapsed or refractory B-cell NHL and CLL. The study will consist of a dose-escalation stage(Phase 1), an expansion stage(Phase 2a) and Phase 2b stage where participants will be enrolled into indication-specific cohorts.

Inclusion Criteria:

1. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
2. CD20+ B-cell Non-Hodgkin Lymphoma or CLL who have relapsed or failed to respond to at least one prior treatment regimen and for whom there is no available therapy expected to improve survival
3. Adequate hepatic, hematologic, and renal function

Exclusion Criteria:

1. Burkitt lymphoma, lymphoplasmacytic lymphoma or B lymphoblastic leukemia
2. Prior treatment with systemic anti-lymphoma therapy within 4 weeks or five half-lives of the drug (which is shorter) prior to the first GB261 infusion
3. History of auto-SCT or CAR-T therapy in the past 180 days and/or with any of protocol specified conditions
4. Prior allo-SCT or allogeneic CAR-T
5. Prior solid organ transplantation
6. Autoimmune disease with the exceptions specified in the protocol
7. History of central nervous system(CNS) lymphoma or other CNS disease
8. Significant cardiovascular or pulmonary disease
9. Hepatitis B or C or human immunodeficiency virus (HIV)
10. Pregnant or lactating or intending to become pregnant during the study