There are a lot of internal changes removing assumptions, correcting bugs etc. There are also a couple of changes that mean that tools and target sets have a different initialization, effectively an API change. Also the deprecated task closure has actually been removed. All the details are to be found in the release notes.
As ever distributions are available at http://dist.codehaus.org/gant. For those who use Maven 2, the repository URL is http://repository.codehaus.org, the groupId is org.codehaus.gant, the artifactId is gant , and the version is 1.4.0.
The Gant webpage has the documentation.