• Documented informed consent of the participant and/or legally authorized representative.

• Confirmed histologic diagnosis of one of the following solid tumors of GI origin:

∙ Gastric adenocarcinoma Note: for gastric adenocarcinoma patients only, central laboratory confirmation of CDH17+ tumor expression is required.

‣ Colon and/or rectal adenocarcinoma

‣ G1, G2, and well-differentiated G3 neuroendocrine tumors of the midgut and hindgut (ileal, jejunal, cecal, distal colonic, or rectal; with ≤ 55% Ki67 expression)

• Availability of unstained tumor tissue slides from archived tumor tissue or a new tumor biopsy, if medically feasible. Note: for gastric adenocarcinoma patients only, confirmation of CDH17+ is required prior to study inclusion.

• Have received at least 1 prior line of systemic anti-cancer treatment in the locally advanced or metastatic setting, as defined by National Comprehensive Cancer Network (NCCN) guidelines. Participants must have received or declined FDA-approved and available treatment options, including targeted therapies for disease mutation or antigen expression status.

• Age ≥ 18 years and ≤ 85 years.

• For Phase 1 Dose Expansion and Phase 2 only: Measurable disease as per RECIST v1.1 criteria (Note: Measurable disease is NOT required for Phase 1 Dose Escalation).

• Eastern Cooperative Oncology Group (ECOG) ≤ 1.

• Life expectancy ≥ 12 weeks.

• No known contraindications to leukapheresis, cyclophosphamide, fludarabine, or steroids.

⁃ Baseline laboratory values as shown in the following table:

⁃ Minimum Laboratory Values for Study Entry Laboratory Assessment Criteria White blood cell count \> 4,000/mm3 Absolute neutrophil count (ANC) ≥ 1,500/mm3 Platelets ≥ 100,000/mm3 Hemoglobin ≥ 10 g/dL Total bilirubin ≤ 1.5 x upper limit of normal (ULN) Aspartate amino transferase (AST) ≤ 3 x ULN Alanine transaminase (ALT) ≤ 3 x ULN Creatinine clearance by Cockroft-Gault equation 60 mL/min Oxygen saturation ≥ 92% on room air Albumin ≥ 3 g/dL

⁃ Left ventricular ejection fraction ≥ 50%.

⁃ Seronegative for human immunodeficiency virus (HIV) by antigen/antibody (Ag/Ab) testing.

⁃ Seronegative for hepatitis B and/or hepatitis C virus.

⁃ Women of childbearing potential (WOCBP) must have a negative urine or serum pregnancy test. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test is required.

⁃ Agreement by women and men of childbearing potential to use an effective method of birth control or abstain from heterosexual activity through at least 3 months after the last dose of CHM-2101.