Q: When starting gnatsd manually (on the command line), I get 520 You are not on the host access list: stdin (stdin).
A: (gnats 3.1xx) Gnatsd is not intended to be started manually, but via inetd, xinetd, or a similar "super server". When gnatsd is started this way, its stdin and stdout (standard input and standard output streams) are connected to a so called "TCP socket" (one end of the network connection), and from this socket gnatsd learns the IP address of the remote end (and from this it derives the remote host name). When you start gnatsd from the command line, its stdin and stdout are connected to the terminal, and thus gnatsd gets a nonsense value.
You usually need root privileges in order to change the configuration of your "super server". If you don't have them, contact your system administrator.
See Starting gnatsd for inetd and xinetd configuration.
A: (gnats 4.0) This error message should not occur with gnatsd 4.0 anymore, because gnatsd can be started from the command line. However, this probably only makes sense for debugging and diagnosing problems. Furthermore, you need to play the part of the client program yourself (i.e., you must "speak" the gnatsd client protocol).