Participants must meet all the following inclusion criteria to be eligible for the study:

1. Participants with pathologically diagnosed locally advanced unresectable or metastatic gastric cancer/gastroesophageal junction adenocarcinoma (GC/GEJA).
2. ≥18 years of age at the time of starting the first dose of T-DXd and signing informed consent form (ICF), capable of providing informed consent.
3. HER2-positive status (IHC 3+ or IHC 2+/ISH +).
4. Received prior anti-cancer treatment regimens according to National Medical Products Administration indication or clinical judgement of physician.
5. Decision to newly initiate T-DXd before study ICF signing. If the participants have started the first dose of T-DXd no longer than 21 days before enrollment, they could be enrolled if the signed and dated ICF could be obtained.

Exclusion Criteria：

Participants who meet any of the following criteria will be excluded from the study:

1. Pregnancy or breastfeeding.
2. Participants who at the time of data collection of the study are participating in or have participated in an interventional study that remains blinded.
3. Known hypersensitivity to either the drug substances or inactive ingredients in the drug product.
4. Judged by the investigator to be unfit to participate in the study.

History of severe allergic reactions to study medication Currently pregnant or breastfeeding Recent participation in another clinical trial within the last 30 days Presence of uncontrolled medical conditions that could affect safety