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

Part 1: Biomarker evaluation/screening phase Primary Objectives: * Evaluate cerebrospinal fluid (CSF) biomarkers in adult Gaucher disease Type 3 (GD3) participants that distinguish GD3 from adult Gaucher disease Type 1 (GD1) participants * Screen adult GD3 participants who qualify for treatment with venglustat in Parts 2, Part 3, and Part 4 Parts 2 and 3: Combination treatment phases Primary objectives: * Evaluate short-term (Part 2) and long-term (Part 3) safety and tolerability of venglustat in combination with Cerezyme in adult GD3 participants * Evaluate the change in CSF central nervous system (CNS) biomarkers (glucosylceramide \[GL-1\] and lyso-glucosylceramide \[lyso-GL-1\]) from adult GD3 participants receiving venglustat in combination with Cerezyme (Part 2 only) Part 4: Extended treatment phase with monotherapy Primary objectives: • Evaluate safety and tolerability of venglustat monotherapy in adult GD3 participants who have remained systemically stable on venglustat in combination with Cerezyme Parts 2 and 3: Combination treatment phases Secondary Objectives: * Evaluate the pharmacokinetics (PK) of venglustat in adult GD3 participants * Evaluate the efficacy of venglustat in combination with Cerezyme in systemic disease in adult GD3 participants by assessing spleen volume, liver volume, hemoglobin level and platelet count * Evaluate the efficacy of venglustat in combination with Cerezyme on neurological function in adult GD3 participants by assessing Ataxia using the Scale for the Assessment and Rating of Ataxia (SARA) * Evaluate plasma biomarkers (lyso-GL-1 and GL-1) in adult GD3 participants Part 4: Extended treatment phase with monotherapy Secondary objectives: * Evaluate the efficacy of venglustat in systemic disease in adult GD3 participants by assessing spleen volume, liver volume, hemoglobin level and platelet count * Evaluate the efficacy of venglustat on neurological function in adult GD3 participants by assessing Ataxia using the Scale for the Assessment and Rating of Ataxia (SARA) * Evaluate plasma biomarkers (lyso-GL-1 and GL-1) in adult GD3 participants

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 \3 years prior to randomization). * Liver volume \