Mac OS X users can get Gant, and Groovy, using MacPorts. As at 20112013-0501-04 09:22+01:00 28T20:36 this has Gant 1.9.5 and Groovy 18.104.22.168.
(Thanks to René Groeschke for updates to the MacPorts system.)
as at 20112013-1101-10 17:11:10+00:0028T20:41. Gant always depends on the Groovy package, but this will get installed automatically if not already installed.
If you want to stay more up to date but don't want to install from source, there is the following deb file which creates a stand-alone installation of Gant with all the dependencies it needs, including Groovy 2.01.0:
Gant 1.9.8 9 standalone
The page https://admin.fedoraproject.org/pkgdb/acls/name/gant details all the Gant packages for the various Fedora versions. Sadly it seems they haven't progressed beyond 1.8.1. If there are any Fedora users out there who can contact the Fedora packagers to get them to update to the latest version that would be good.