Evaluating Long-term Safety of Efgartigimod Administered Intravenously and Efgartigimod PH20 Administered Subcutaneously in Children With Generalized Myasthenia Gravis

The purpose of this trial is to evaluate the long-term safety of efgartigimod IV and efgartigimod PH20 SC administered to participants with gMG in the antecedent studies, ARGX-113-2006 and ARGX-113-2207, respectively. Participants will receive efgartigimod IV or efgartigimod PH20 SC, using the dose administered in the antecedent studies. Participants who have not reached the age of 18 can remain in the study until efgartigimod becomes commercially available in the respective country or available through another continued access program for gMG. Participants who have reached the age of 18 can remain in the study for a maximum of 2 years, until efgartigimod becomes commercially available in the respective country or available through another continued access program for gMG, whichever comes first.

Inclusion Criteria: * The participant completed ARGX-113-2006, defined as: 1. The participant reached End of Trial in trial ARGX-113-2006 or End of Study in ARGX-113-2207 and agreed to participate in the ARGX-113-2008 trial. 2. The participant qualifies for retreatment in trial ARGX-113-2006, but cannot complete a Treatment Period (TP) and the required Intertreatment Period (IP) visits within the ARGX-113-2006 trial's timeframe. * Either the participant or the participant's legally authorized representative can understand the requirements of the trial and provide written informed consent/assent, and willingness and ability to comply with the trial protocol procedures. * Contraceptive use for sexually active participants should be consistent with local regulations for those participating in clinical studies. Exclusion Criteria: * Female adolescents of childbearing potential (FAOCBP): Pregnancy or lactation, or the participant intends to become pregnant during their participation in the study. * Discontinued early from ARGX-113-2006 or ARGX-113-2207 treatment. * Clinically significant uncontrolled chronic bacterial, viral, or fungal infection at study entry not sufficiently resolved in the investigator's opinion or known autoimmune disease or any medical condition that would interfere with an accurate assessment of clinical symptoms of gMG or put the participant at undue risk.