Google used to tell you how to do this on their website. I cant find it now, so maybe they are no longer supporting this. However, at the time of writing this, it still works for syncing contacts at least.

On the phone, go to Organizer->Synchronization->New Account

Give it a name, such as “Google Sync”

Server Address	https://m.google.com/syncml
Username	Your google account username
Password	Your google account password
Connection	Your Connection Profile
Applications	Contacts

Then under App. settings, select Contacts and then for Database name use contacts

That should be it. Now you can set it to sync on a schedule. Don’t forget to back up your contacts on both your Google account and on your phone first just in case it all goes wrong.