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 late-onset Pompe disease （LOPD） who are ≥ 6 years old will be studied.

Inclusion Criteria:

• Age ≥ 6 years, males or females;
• Patient has a diagnosis of LOPD;
• Patient has upright FVC ≥ 30% of predicted normal value;
• A 6MWT ≥ 40 meters, assistive device allowed;
• 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:

• Patient who has any history or concurrent clinical organic disease, including cardiovascular and liver diseases, respiratory system, nervous system disease, or any other condition that, in the opinion of the investigator, makes the subject unsuitable for participation in the study.
• Patient who requires invasive mechanical ventilation, or rely on noninvasive non-non-invasive assisted ventilation when sitting upright;
• Patient who is positive for human immunodeficiency (HIV) antibody, hepatitis B surface antigen, hepatitis C antibody, or treponema pallidum antibody;
• Patient with a history of glucocorticoid allergy;
• Patient who has a contraindication to study drug or to corticosteroids, or has demonstrated hypersensitivity to any of the components of the study drug;
• Patient who has AAV9 neutralizing antibody titer ≥ 1:100；
• Patient who has participated in a previous gene therapy research trial;
• Pregnant or lactating female participants;
• Patients who have fertility plans within 6 months from screening to the end of the study and are unwilling to take effective physical contraceptive measures (such as a condom, intrauterine device, contraceptive ring, ligation, abstinence, etc.) for contraception (including the subject's partner);