This is a NFSv2-specific option.
The mapping daemon ugidd must be running.
Like Don't trust UIDs, this option
allows you to specify a list of client GIDs to be treated as the
untrusted group. You must enter a comma-separated list
of GIDs or GID ranges like 1,10,20-25,100-150.
exportfs options: squash_gids, map_daemon