The goal of this clinical trial is to learn if Omega-3 fatty acids supplementation can enhance the efficacy of neoadjuvant immunotherapy in patients with locally advanced gastric and gastroesophageal junction adenocarcinoma. The main questions it aims to answer are: does Omega-3 supplementation improve the pathologic complete response (pCR) rate compared to standard neoadjuvant immunotherapy alone, and is there an increase in the major pathological response (MPR) rate with Omega-3 supplementation? Researchers will compare the group receiving Omega-3 supplementation with the control group receiving standard neoadjuvant immunotherapy to see if Omega-3 supplementation leads to better treatment outcomes. Participants will receive Omega-3 fatty acids supplements in conjunction with their prescribed neoadjuvant immunotherapy, undergo regular assessments of their pathologic response to treatment, and provide information on their nutritional status and quality of life throughout the study.

Inclusion Criteria:

1. Age: Participants must be aged 18 years or older.

2. Diagnosis: Confirmed diagnosis of locally advanced gastric or gastroesophageal junction adenocarcinoma, classified as clinical stage II or III according to the American Joint Committee on Cancer (AJCC) staging system.

3. Performance Status: Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2, indicating that patients are fully active or restricted in physically strenuous activity but are capable of self-care.

4. Histological Verification: Tissue confirmation of diagnosis must be obtained via biopsy, with adequate archival tumor tissue available for biomarker analysis if required.

5. Organ Function: Adequate organ function as evidenced by laboratory tests:

   Absolute neutrophil count (ANC) ≥ 1,500/mm³ Platelet count ≥ 100,000/mm³ Hemoglobin ≥ 9 g/dL Serum creatinine ≤ 1.5 times the upper limit of normal (ULN) Total bilirubin ≤ 1.5 times the ULN (unless due to Gilbert's syndrome) Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 times the ULN (≤ 5 times the ULN if liver metastases are present)

6. Informed Consent: Ability and willingness to provide written informed consent prior to any study-related procedures.

Exclusion Criteria:

1. Previous Therapy: Prior treatment with chemotherapy, radiation therapy, or immunotherapy for the current cancer diagnosis.
2. Concurrent Malignancies: Presence of another active malignancy within the past 5 years (except for non-melanoma skin cancer or cervical carcinoma in situ).
3. Active Infection: Presence of any uncontrolled infection requiring systemic therapy or serious co-morbid conditions that would compromise program compliance or interfere with study results.
4. Allergies: Known hypersensitivity or contraindications to Omega-3 fatty acid supplementation or components of the immunotherapy regimen.
5. Pregnancy/Breastfeeding: Women who are pregnant or breastfeeding; fertile men and women not employing effective contraception during the study period.

Psychiatric Condition: Any psychiatric condition or substance abuse that would interfere with compliance or affect the participant's safety.