This randomized, multicenter, double-blind, placebo-controlled study will evaluate the efficacy and safety of onartuzumab (MetMAb) in combination with 5-fluorouracil, folinic Acid, and oxaliplatin (mFOLFOX6) in participants with metastatic human epidermal growth receptor (HER) 2-negative and MET-positive adenocarcinoma of the stomach or gastroesophageal junction. Participants will be randomized in a 1:1 ratio to receive either onartuzumab or placebo in combination with mFOLFOX6. Participants may continue to receive onartuzumab or placebo until disease progression, unacceptable toxicity, participant or physician decision to discontinue treatment.