Group Management Services (GMS), a certified professional employer organization (CPEO), was started in 1996 and is headquartered in Richfield, Ohio. At GMS, our mission is to provide our clients with the HR tools and resources vital to help their businesses grow and succeed. Representing over 40,000 worksite employees, GMS prides itself on its people first approach – meaning, no automated voice systems or chat bots attempting to answer your question.