The git:// protocol operates on port 9418 (documented in the git-daemon man page, or run git daemon --help). To forward this via an SSH tunnel, you would do something like this:
ssh -L 9418:gitorious.org:9418 your.remote.host
Once connected, you would clone a remote repository like this:
git clone git://localhost/path/to/repository.git
In order for this to work you obviously need a machine outside your firewall to be the target of the ssh command.
