This is a multicenter, randomized, double-blind, placebo-controlled, parallel-group, phase 2 study. A total of 276 eligible Parkinson's disease participants with motor fluctuations will be enrolled and assigned to one of three cohorts. Within each cohort, participants will be randomized in a 3:1 ratio to receive either GM1 or matching placebo, resulting in six groups: Cohort 1 GM1 (n=69), Cohort 1 Placebo (n=23); Cohort 2 GM1 (n=69), Cohort 2 Placebo (n=23); Cohort 3 GM1 (n=69), Cohort 3 Placebo (n=23). All participants will continue their pre-enrollment anti-Parkinson's medication regimen as background therapy, which should remain as stable as possible during the study. The study consists of a 28-day screening period and an 85-day double-blind treatment period.