This is a Multicenter, Randomized, Open-label, Phase 3 Study of IBI343 Monotherapy Versus Treatment of Investigator's Choice in Subjects with Previously Treated Claudin (CLDN) 18.2-positive, HER2-negative, Locally Advanced, Unresectable or Metastatic Gastric or Gastroesophageal Junction Adenocarcinoma to compare the progression free survival (PFS) and overall survival (OS)

Inclusion Criteria:

1. Able and willing to sign a written Informed Consent Form(ICF) and to comply with protocol-specified visits and related procedures.
2. Has histopathologically confirmed unresectable locally advanced or metastatic adenocarcinoma of the gastric/gastroesophageal junction (G/GEJ AC).
3. Has received and progressed on at least 2 lines of systemic therapy (anti-PD-(L)1 in combination with platinum or fluoropyrimidines, paclitaxel/docetaxel, irinotecan). A prior (neo)adjuvant systemic therapy that ended within 6 months prior to disease relapse is defined as the first line therapy. The subject has ≤ 4 prior lines of systemic therapy.
4. Has histopathologically confirmed CLDN18.2-positive disease.
5. Is a man or woman of 18 years of age or older at the time of signing the ICF.
6. Has an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1.

Exclusion Criteria:

1. Has HER2-positive (defined as immunohistochemistry [IHC] 3+, or IHC 2+ and positive by in situ hybridization) disease.
2. Is currently participating in another interventional clinical study, except when the subject is during survival follow-up of an interventional clinical study.
3. Has a history of treatment with topoisomerase inhibitorbased antibody-drug conjugate(s).
4. Has received the last dose of an anti-cancer therapy (including traditional Chinese medicine indicated for gastric cancer in the package insert, but excluding herbal prescriptions) within 4 weeks or 5 half-lives (whichever is shorter) prior to the first dose of study treatment.
5. Plans to receive other anti-cancer therapy during treatment with the study drug (palliative radiotherapy for symptomatic (e.g., pain) relief that does not affect response assessment is allowed).