Study to Evaluate the Efficacy and Safety of Venglustat in Adult and Pediatric Patients With Gaucher Disease Type 3

Inclusion Criteria: * The participant has received ERT (Cerezyme or other ERT; as deemed appropriate by local regulations) for at least 3 years prior to enrollment, on a stable dose for at least 6 months, is deemed clinically stable for at least 1 year by the Investigator and is within the therapeutic goals as all of the following: * Hemoglobin level of ≥11.0 g/dL for females and ≥12.0 g/dL for males * Platelet count ≥100 000/mm3 * Spleen volume \<10 multiples of normal (MN) * Liver volume \<1.5 MN * No bone crisis and free of symptomatic bone disease such as bone pain attributable to osteonecrosis and/or pathological fractures within 3 months prior to screening * Adult participant is ≥18 years of age * Pediatric participant is ≥12 years \<18 years of age * The participant has a clinical diagnosis of GD3 and a documented deficiency of acid beta-glucosidase activity confirming this diagnosis. * The participant has a modified SARA score of 1 or above. * The presence of gaze palsy, predominantly horizontal, with slow or absent saccades. * If the participant has a history of seizures, they are well controlled under appropriate medication not identified as a strong or moderate inducer or inhibitor of CYP3A. * Participants ≥ 30 kg of weight * Contraception for sexually active male or female participants; not pregnant or breastfeeding; no sperm donating for male participant * Signed written informed assent/consent Exclusion Criteria: * The participant is blood transfus…