The only way to install Google Mobile Services apps including the Play Store on GrapheneOS is as regular sandboxed apps. They aren't granted any of the large number of usual privileged permissions, don't run with the usual far more privileged SELinux policies, aren't used by the OS as a backend for anything and aren't otherwise allowed to do special things by the OS in the usual many ways that is granted. They're regular sandboxed apps on GrapheneOS.