The latest update to Mac OSX 10.5.3 now includes inbuilt google/address book synchronization.
However - after updating and adding my login details for gmail to addressbook it just seemed to hang with:
Can't connect to the sync server: NSInvalidReceivePortException: connection went invalid while waiting for a reply ((null))
The solution was to remove the contents of
~/Library/Application Support/SyncServices/Local
and try again.
Note that for account name it needed the full e-mail address complete with googlemail.com domain.
Update
I use a plugin to Aperture to send mail to gmail. It was failing due to a
javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target