The source for Gant resides in a Git repository. The mainline is held on GitHub so as to make use of all the forking and pull request technology:
There is a repository on Codehaus but this is an administrative mirror of the repository on GitHub.
Anonymous non-committer access is allowed, but should be avoided in favour of using the GitHub resources.
Should only be used to undertake administrative activity, not development activity, and even then all activity should happen through GitHub.
The source repository can be browsed online, but it is probably better to go to the GitHub repository.