• Voluntary agreement to provide written informed consent.

• Male or female, Age ≥ 18 years.

• Predicted survival ≥ 12 weeks.

• Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1.

• All female subjects will be considered to be of child-bearing potential unless they are postmenopausal, or have been sterilized surgically.Female subjects of child-bearing potential must agree to use two forms of highly effective contraception. Male subjects and their female partner who are of child-bearing potential must agree to use two forms of highly effective contraception.

• Willing to adhere to the study visit schedule and the prohibitions and restrictions specified in this protocol.

• Adequate organ function.

• All subjects must have inoperable, advanced or metastatic gastric or or gastroesophageal adenocarcinoma

• Have had progression or intolerance following receipt of at least two systemic chemotherapy for advanced or metastatic disease.

• The HER2 IHC test result is IHC 2+ or IHC 3+, the subject's previous test results (confirmed by the investigator) or the research center's test results are acceptable; the subject can provide the Specimen of primary or metastatic tumor for HER2 review/judgment.

• HER2 classic positive (definition: IHC3+ or IHC2+FISH+) and patients who have previously failed standard treatment

• According to the RECIST 1.1 standard, there is at least one measurable lesion.