A Study to Evaluate the Safety and the Activity of S095029 as Part of Combination Therapy in Advanced Gastroesophageal Junction/Gastric Cancers.

Inclusion Criteria:

• Have a confirmed diagnosis of locally advanced and unresectable or metastaticgastric or gastro-esophageal junction adenocarcinoma

• Participants' tumor must have an MSI-H/dMMR status according to institutionalguidelines and/or according to the College of American Pathologists, determined atany time prior to enrolment.

Exclusion Criteria:

• Has received more than one previous line of treatment in the locally advanced andunresectable or metastatic setting.

• Has received prior therapy with any checkpoint inhibitor (anti-PD-1, anti-programmedcell death ligand 1 (PDL1), anti-CTLA4).

• Participants who have received prior systemic anti-cancer therapy includinginvestigational agents within 4 weeks (shorter interval, at least 5 half-lives, forkinase inhibitors or other short half-life drugs) prior to first study treatment.

• Prior radiotherapy if completed less than 2 weeks before first study treatment

• Major surgery less than 4 weeks prior to the first study treatment or participantswho have not recovered from the side effects of the surgery.