The purpose of this study is to evaluate the efficacy and safety of nipocalimab compared to placebo in participants with generalized myasthenia gravis (gMG). The purpose of the subcutaneous substudy is to evaluate how well it works in the body (pharmacodynamic \[PD\]) when given as an injection under the skin (subcutaneous) compared to when given through a vein (intravenous) in participants with gMG.