The main objective of this study is to compare efficacy of bemarituzumab combined with oxaliplatin, leucovorin, and 5-fluorouracil (5-FU) (mFOLFOX6) to placebo plus mFOLFOX6 as assessed by overall survival (OS) in participants with FGFR2b ≥10% 2+/3+ tumor cell staining (FGFR2b ≥10% 2+/3+TC)