Additional Study Information: This study is being conducted in participants with gastric (stomach) cancer or cancer of the astroesophageal junction (where the esophagus meets the stomach) that has returned after treatment (recurrent), or has spread to another area of your body (metastatic). The main purpose is to determine the doses and dosing schedule of both MEDI4736 and tremelimumab that are safe and tolerable when given together in subjects with gastric (stomach) cancer or cancer of the gastroesophageal junction (where the esophagus meets the stomach). In the second part of this study, adult subjects with metastatic or recurrent gastric cancer or cancer of the gastroesophageal junction will be randomly enrolled into 1 of 4 treatment groups based on previously received treatments for their cancer