The purpose of this study is to test and find a safe dose of this investigational gene therapy PBGM01 (also known as, the “study drug”) in children who have been diagnosed with the early infantile or late infantile form of GM1. The study will also try to determine how PBGM01 might affect the GM1 disease course over time. The study drug is investigational, which means it has not been approved for marketing by the FDA. This study is the first time PBGM01 will be given to humans, so we do not know for sure if participants will benefit from this study.