The gsscred table is used by an NFS server when the server is trying to identify a Kerberos user, if the default mappings are not sufficient. The NFS service uses UNIX IDs to identify users. These IDs are not part of a user principal or a credential. The gsscred table provides additional mapping from GSS credentials to UNIX UIDs (from the password file). The table must be created and administered after the KDC database is populated. See Mapping GSS Credentials to UNIX Credentials for more information.
When a client request comes in, the NFS service tries to map the credential name to a UNIX ID. If the mapping fails, the gsscred table is checked.