GoGui can be invoked with the command:
java -jar gogui.jar [options] [program]
See gogui(1) for a complete listing of the possible options to gogui.
For Unix systems there are launcher scripts in the bin directory. The launcher scripts respect the environment variable JAVA_HOME, if defined, and look for the jar files in ../lib relative to the bin directory. You can add the bin directory of GoGui to your PATH environment variable. The invocation is then simply:
gogui [options] program