Git.pmis self-contained, so copy it to the directory part of your personal perl @INC. This requires Error.pm(with a version greater than or equal to 0.15009), so be sure to also install this module.
If you are looking for a Git shell (and not specifically Git.pm), several CPANs are available (in alphabetical order):
Git::ClassGit::RepositoryGit::Wrapper
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