This study is testing a new medicine, BIA 28-6156, to see if it can help slow down Parkinson's disease (PD) symptoms in people with a specific gene change called GBA1. *Parkinson's disease* is a brain disorder that affects movement, causing symptoms like shaking and stiffness. *GBA1* is a gene that, when changed, can increase the risk for this disease. There are two parts to the study: **Part A** checks if participants have the GBA1 gene change, and **Part B** tests the medicine. Participants will either get the medicine (at two different doses) or a placebo, which is a harmless pill. The study will last about 78 weeks, with an additional 4 weeks for follow-up checks. **Key Points:** - The study involves a long-term commitment of 82 weeks, including follow-up. - Participants need to be on stable PD medications before and during the study. - The study is double-blind, meaning neither participants nor the researchers will know who is getting the actual medicine or the placebo. This study could help find better treatments for people with Parkinson’s disease who have the GBA1 gene change.