This is a randomized, multicenter, open-label, dose-exploration and dose-expansion study to evaluate the safety, tolerability, antitumor activity, PK, pharmacodynamics, and immunogenicity of MEDI4736 in combination with tremelimumab, MEDI4736 monotherapy or tremelimumab monotherapy in subjects with metastatic or recurrent gastric or gastroesophageal junction adenocarcinoma