The main purpose of this study is to measure how well adults with new-onset gMG (which means they've had generalized disease signs and/or symptoms for less than 1 year) respond to treatment with efgartigimod PH20 SC. The study consists of a treatment period of 51 weeks. The study duration for each participant will be approximately 58 weeks.

Inclusion Criteria:

• Is at least 18 years when signing the ICF
• Has been diagnosed with gMG of MGFA class II, III, or IV
• Is seropositive for AChR-Ab
• Is treatment-naive for gMG or has been administered AChEI for the treatment of gMG
• Had onset of generalized MG signs and/or symptoms within 12 months before screening; candidates who also had onset of ocular MG signs and/or symptoms within 24 months before screening may be enrolled in the study
• Has an MG-ADL score ≥5

Exclusion Criteria:

• gMG diagnosis of MGFA class I or V
• Underwent a thymectomy prior to screening, except thymectomy for treatment of nonmalignant thymoma prior to the gMG diagnosis
• Prior or current use of any of any systemic corticosteroid therapy or nonsteroidal immunosuppressive therapy for the treatment of gMG