Given that Google's main use case is to get you to abandon on-premise directory services I think you're going to have a tough time finding any canned examples.
The Google Directory API has functionality to retrieve all users and the output should be reasonably easy to parse (since it's just JSON). You might be able to cobble something together with this API and a tool like LDAP Sync Connector. (I don't know enough about that particular tool, but there's a posting on their mailing list that claims that Google Apps can be used as a sync source.)
(Trying to search for this is pretty frustrating. I can find people looking for what you want, but inevitably a bunch of "helpful" people come along and suggest Google Apps Directory Sync, which is exactly not what you or they are looking for.)
@Zoredache raises a very good and practical point. I see no way to get password hashes back from Google. I can't imagine a world in which they'd ever let you download those.
Bear in mind that you're going to have to deal with passwords yourself.
If that's problematic you might look at using Google to do the authentication and using OAuth2 (though I'm not readily finding a PAM module).