In 3.xxx versions, gnatsd uses port 1529 by default; as this port is officially assigned to another application (see the list of port numbers maintained by the Internet Assigned Numbers Authority) it will probably change in 4.x.
If you want/need to run gnatsd on another port, you can change the
default port at compile time (call
configure --help to learn
about compile time configuration options). Most clients also accept
an option or configuration variable to change the port. Don't forget
to tell inetd (or xinetd, or whatever super-server you use) to start
gnatsd on the other port.