The GMS parent cooperative is central to the school's original vision and essential to its current operation. As a system of work-sharing, in-kind contribution and community building, the co-op enables GMS to keep tuition low and keep running costs lower. It also expands opportunities for family involvement and provides a rich array of resources and services.

Each parent or guardian is required to contribute 40 hours over the course of each year, usually in the form of a particular co-op "job". Typical "jobs" cover a range of needed services—from building maintenance to lunchtime supervision, from coordination of special events to substituting for office personnel, from cataloging library books to participating in curriculum development. The Co-Op Manager works with the school staff and parent body to equitably organize, assign, and oversee these jobs.