You will want to add a user entry to your /etc/passwd file for the
user-id that owns gnats-related databases on your server.
Something of the form:
   "gnats:*:61:61:GNATS database owner:${PREFIX}/share/gnats/gnats-db:"
should do just fine (put it in your ftpusers file!).

Don't forget to run ${PREFIX}/libexec/gnats/mkcat as gnats.