Protecting Access to Ground Ambulance Medical Services Act of 2021 This bill modifies Medicare payment of ground ambulance services in rural areas. Specifically, the bill extends the specialized rate increase for such services for an additional five years (until January 1, 2028). The bill also requires the Centers for Medicare & Medicaid Services to (1) allow an area that has 1,000 or fewer individuals per square mile to retain its status as a rural area even if it would otherwise no longer qualify, and (2) establish an appeals process for providers to seek reconsideration of a change in an area's status.