GMT has not been ported to the classical Macintosh platform (i.e. MacOS 8.x). For that OS your only option is MachTen. However, GMT will install directly under the MacOS X Server. The MacOS X Server was released in March 1999 and is built on a BSD UNIX implementation that will allow users to install GMT or any other UNIX package. The server version costs USD 499 (USD 299 for academics). On the other hand, the MacOS X user version (i.e., the version installed on new Macs you may purchase) will not be ready until late 1999 and may or may not provide a command prompt. You may find out the status of these issues by checking the latest news at macUNIX.