This study is being conducted to evaluate the safety and effectiveness of GC301 adeno-associated virus vector expressing codon-optimized human acid alpha-glucosidase (GAA) as potential gene therapy for Pompe disease. Patients diagnosed with infantile-onset Pompe disease who are younger than 6 months old will be studied.

Inclusion Criteria:

• Age < 6 months
• Patient has diagnosis of infantile onset Pompe disease
• The patient's legal guardian(s) must be able to understand the purpose and risks of the study and voluntarily provide signed and dated informed consent prior to any study-related procedures being performed.

Exclusion Criteria:

• Left ventricle ejection fraction (LVEF) < 40%;
• Patient who has AAV9 neutralizing antibody titer ≥ 1:100；
• Patient who has received enzyme replacement therapy (ERT) more than twice;
• Patient who has respiratory dysfunction before enrollment, including the blood oxygen (O2) saturation level < 90%, or the partial pressure of carbon dioxide (PCO2) in venous blood > 55 mmHg, or PCO2 in arterial blood > 40 mmHg;
• Patient who has laboratory abnormalities of: creatinine > Upper Limit of Normal (ULN), hemoglobin < 90 g/L;
• Patient with congenital organ absence;
• Patient with a history of glucocorticoid allergy;
• Patient who is positive for human immunodeficiency (HIV) antibody, hepatitis B surface antigen, hepatitis C antibody, or treponema pallidum antibody;
• Patient who has participated in a previous gene therapy research trial;
• Patient who has any concurrent clinically significant major disease or any other condition that, in the opinion of the investigator, makes the subject unsuitable for participation in the study.