Venglustat in Combination With Cerezyme in Adult Patients With Gaucher Disease Type 3 With Venglustat Monotherapy Extension

Inclusion Criteria: GD3 and GD1 participants must meet the following criteria to be eligible for this study: * GD1 participant is ≥18 and ≤40 years of age. * GD3 participant is ≥18 years of age. * Participant must provide written informed consent prior to any study-related procedures being performed. * Participant has a clinical diagnosis of Gaucher disease Type 1 (GD1) or Gaucher disease Type 3 (GD3) and documented deficiency of acid beta-glucosidase activity confirming this diagnosis. * 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 and is within the therapeutic goals defined below, and is deemed clinically stable for at least 1 year by the Investigator. * Participant has reached Gaucher disease therapeutic goals defined as all of the following to be eligible for this study: * 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), or total splenectomy (provided the splenectomy occurred \>3 years prior to randomization). * 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. * Participant has maintained GD therapeutic goals defined as all of the following to be eligible for entering Part 4 of this study: * 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), or total splenectomy * 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 entering Part 4 * Participant, if female and of childbearing potential, must have a negative pregnancy test \[urine beta-human chorionic gonadotropin (β-hCG)\] at baseline. * If participant has a history of seizures, except for myoclonic seizures, they are well controlled under appropriate medication not identified as a strong or moderate inducer or inhibitor of cytochrome P450 (CYP) 3A. * Participant is willing to abstain from consumption of grapefruit, grapefruit juice, or grapefruit containing products for 72 hours prior to administration of the first dose of venglustat and for the duration of the treatment period. * Oculomotor apraxia characterized by a horizontal saccade abnormality. * Female participants of childbearing potential and male participants must be willing to practice true abstinence in line with their preferred and usual lifestyle, or use 2 acceptable effective methods of contraception for the duration of the study and for at least 6 weeks for females and 90 days for males following their last dose of venglustat. Exclusion Criteria: Participants are excluded from the study if any of the following criteria apply: * Substrate reduction therapy or chaperone therapy for GD within 6 months prior to enrollment. * Participant has had a partial or total splenectomy within 3 years prior to randomization. * Participant is blood transfusion-dependent. * Prior esophageal varices or liver infarction or current liver enzymes (alanine aminotransferase \[ALT\]/ aspartate aminotransferase \[AST\]) or total bilirubin \>2 times the upper limit of normal, unless the participant has a diagnosis of Gilbert Syndrome. * Participant has any clinically significant disease, other than GD, including cardiovascular (congenital cardiac defect, coronary artery disease, valve disease or left sided heart failure; clinically significant arrhythmias or conduction defect), hepatic, gastrointestinal, pulmonary, neurologic, endocrine, metabolic (eg, hypokalemia, hypomagnesemia) or psychiatric disease, other medical conditions, or serious intercurrent illnesses that may preclude participation. * Participant has renal insufficiency, as defined by an estimated glomerular filtration rate \<30 mL/min/1.73m2 at the screening visit. * Participant has received an investigational product within 30 days prior to enrollment. * Participant has a history of cancer, with the exception of basal cell carcinoma. * Participant has myoclonic seizures. * Participant is pregnant or lactating. * Participant has, according to World Health Organization (WHO) Grading, a cortical cataract \> one-quarter of the lens circumference (Grade cortical cataract-2) or a posterior subcapsular cataract \>2 mm (Grade posterior subcapsular cataract-2). Participants with nuclear cataracts will not be excluded. * Participant requires use of invasive ventilatory support. * Participant requires use of noninvasive ventilator support while awake for longer than 12 hours daily. * Participant is unable to receive treatment with Cerezyme due to a known hypersensitivity or is unwilling to receive Cerezyme treatment to ensure maintenance of Gaucher treatment goals. * Participant is currently receiving potentially cataractogenic medications (corticosteroids, psoralens used in dermatology with ultraviolet light therapy \[PUVA\], typical antipsychotics, and glaucoma medications) or any medication that may worsen the vision of a participant with cataract (eg, alphaadrenergic glaucoma medications). * Participant has received strong or moderate inducers or inhibitors of CYP3A within 15 days or 5 half-lives from screening, whichever is longer, prior to enrolment in Part 2. This also includes the consumption of grapefruit, grapefruit juice, or grapefruit containing products within 72 hours of starting venglustat administration in Parts 2 and 3. * Participant is scheduled for in-patient hospitalization including elective surgery, during the study. * Participant has had a major organ transplant (e.g., bone marrow or liver). * Participant, in the opinion of the investigator, is unable to adhere to the requirements of the study or unable to undergo study assessments (e.g., contraindications for magnetic resonance imaging).