* INCLUSION CRITERIA: Type I subjects * Male or female subjects \>= 6 months old and \<= 12 months old at time of full ICF signing * Biallelic mutations in GLB1 * Documented deficiency of Beta-galactosidase enzyme by clinical laboratory testing * Phenotype consistent with a diagnosis of Type I GM1 gangliosidosis * Symptomatic subjects: as determined by the opinion of the Principal Investigator and based on the criteria set forth by Brunetti-Pierri et al: * Age of symptom onset \<= 6 months of age * Rapidly progressive with developmental delay and hypotonia * Pre- symptomatic subjects: must have mutations confirmed to be associated with the Type I subtype * AAV9 antibody titers \<=1:50 * Agree to reside within 50 miles of the study site for at least 1 month following treatment Type II subjects * Vineland-3 Adaptive Behavior composite standard score greater than or equal to 40 * Male or female subjects \> 6 months old and \< 12 years old at time of full ICF signing * Biallelic mutations in GLB1 * Documented deficiency of beta-galactosidase enzyme by clinical laboratory testing * Phenotype consistent with a diagnosis of Type II GM1 gangliosidosis, with symptom onset after the first year of life * AAV9 antibody titers \<=1:50 * Agree to reside within 50 miles of the study site for at least 1 month following treatment EXCLUSION CRITERIA: * AAV9 antibody titers \>1:50 * Contraindications to concomitant medications * Serious illness that would not allow travel to the study site * Unwilling to undergo study interventions as outlined in the Schedule of Events * Subjects receiving other unapproved, off-label or experimental therapies for GM1 gangliosidosis (i.e. miglustat, Tanganil) within the last 60 days * Any prior participation in a study in which a gene therapy vector or stem cell transplantation was administered * Pregnant or lactating subjects * Immunizations of any kind in the month prior to screening * Evidence of cardiomyopathy on history, exam, or additional testing (echocardiogram or electrocardiogram) or other cardiac disease that in the opinion of the investigator would deem the subject unsafe to participate in the trial * Indwelling ferromagnetic devices that would preclude MRI/fMRI/MRS imaging * Ongoing medical condition that is deemed by the Principal Investigator to interfere with the conduct or assessments of the study * History of infection with human immunodeficiency virus (HIV), hepatitis A, B, or C, or tuberculosis. * History of or current chemotherapy, radiotherapy or other immunosuppressive therapy within the past 30 days. Corticosteroid treatment may be permitted at the discretion of the PI * Abnormal laboratory values considered clinically significant per the investigator * Failure to thrive, defined as: \-- Falling 20 percentiles (20/100) in body weight in the 3 months preceding Screening/Baseline * Underlying defect in immune function * History of multiple and severe life-threatening infections