Previously released versions of the code are available, but it is recommended to use the most recent version.
The current version (2007-07-25) of the gpml code is available here.
Changes bewteen versions are documented here.
The version dated 2007-06-25 is available as gpml-matlab-2007-06-25.tar.gz and gpml-matlab-2007-06-25.zip.
The version dated 2006-09-08 is available as gpml-matlab-2006-09-08.tar.gz and gpml-matlab-2006-09-08.zip.