This is a multicenter, phase 2, open label study to evaluate safety, tolerability and efficacy of SHR2554 combined with other anti-tumor treatments in patients with advanced or metastatic gastric or gastroesophageal junction adenocarcinoma.

Inclusion Criteria:

1. At least 18 years or the minimum legal adult age (whichever is greater) at the time the ICF is signed.
2. Has at least 1 measurable lesion based on investigator imaging assessment (computed tomography or magnetic resonance imaging) using RECIST v1.1 at screening.
3. Is willing to provide an adequate tumor sample.
4. Eastern Cooperative Oncology Group performance status (ECOG PS) of 0 or 1 at Screening.

Exclusion Criteria:

1. Presence of dysphagia or other factors impairing oral administration of SHR2554.
2. Has previously been treated with any enhancer of zeste homolog inhibitors.
3. Uncontrolled or significant cardiovascular disease.
4. Has spinal cord compression or clinically active central nervous system metastases, defined as untreated and symptomatic, or requiring therapy with corticosteroids or anticonvulsants to control associated symptoms.
5. Has active autoimmune diseases requiring systemic corticosteroids/immunosuppressants.
6. History of known hypersensitivity to SHR2554 or excipients.
7. Evidence of ongoing uncontrolled systemic bacterial, fungal, or viral infection requiring treatment with intravenous (IV) antibiotics, antivirals, or antifungals.
8. Diagnosis of other malignancies within 5 years prior to the first dose of investigational product.
9. Has history of interstitial lung disease (ILD), non-infectious pneumonitis requiring systemic glucocorticoids, current suspected/confirmed ILD, or clinically significant pulmonary disease history.
10. Psychological, social, familial, or geographical factors that would prevent regular follow-up.