The purpose of this study is to measure the pharmacokinetics (PK), pharmacodynamics (PD), safety, tolerability, and immunogenicity of efgartigimod PH20 SC in pediatric participants with gMG aged 2 to <18 years. The primary goal is to confirm an appropriate dose of efgartigimod PH20 SC for pediatric patients using PK and PD results from this study. Participants will receive injections of efgartigimod PH20 SC and will be monitored for safety until the end of the study. At the end of the follow-up period, eligible participants may roll over to an open-label extension (OLE) study.

The participants will be in the study for up to 14 weeks.

Inclusion Criteria:

• The participant (and/or their legally authorized representative) understands the requirements of the study and is capable of providing written informed consent/assent and complying with protocol requirements
• The participant is aged 2 to <18 years at the time of informed consent/assent
• The participant has been diagnosed with generalised Myasthenia Gravis that is supported by a physical examination and confirmed seropositivity for anti-acetylcholine receptor antibodies
• The participant has had an unsatisfactory response to immunosuppressants, corticosteroids, or acetylcholinesterase inhibitors but is on stable concomitant MG therapy. If receiving corticosteroids and/or immunosuppressants, must be on a stable dose for ≥1 month before screening
• The participant agrees to use birth control consistent with local regulations and people of child-bearing potential must have a negative blood pregnancy test at screening and a negative urine pregnancy test before receiving the study drug

Exclusion Criteria:

• Is a female adolescent of child-bearing potential who is pregnant and/or lactating or intends to become pregnant during their participation in the study
• Has worsening muscle weakness secondary to a concurrent infection or as a result of a medication
• Has a documented lack of clinical response to plasma exchange (PLEX)
• Received a live or live-attenuated vaccine within <4 weeks before screening
• Received a thymectomy within 3 months before screening or is planning to get a thymectomy during their participation in the study
• Has a known autoimmune disease or any medical condition that would interfere with an accurate assessment of clinical symptoms of generalised Myasthenia Gravis or puts the participant at undue risk
• History of malignancy, cancer, unless considered cured by adequate treatment with no evidence of recurrence for ≥3 years. Adequately treated participants with the following cancers can be included at any time: Basal cell or squamous cell skin cancer, Carcinoma in situ of the cervix, Carcinoma in situ of the breast, Incidental histological findings of prostate cancer
• Clinically significant active infection that is not sufficiently resolved in the investigator's opinion or positive serum test at screening for active infection with any of the following: Hepatitis B virus (HBV), Hepatitis C virus (HCV), HIV
• Has a positive PCR test for SARS-CoV-2 at screening
• Has/had a clinically significant disease, had recent major surgery (within 3 months of screening) or intends to have major surgery during the study, or has/had any other medical condition that, in the investigator's opinion, would confound the results of the study or put the participant at undue risk
• Has received a different study drug in another clinical study within <12 before screening
• Is currently participating in another interventional clinical study
• Has previously participated in an efgartigimod clinical study and received at least one dose of study drug
• Has a known hypersensitivity to study drug or any of its excipients
• Has a history of or current episode of alcohol, drug, or medication abuse as assessed by the investigator
• Use of some medications before screening (more information is found in the protocol)

The complete list of exclusion criteria can be found in the protocol.