From MacOSx 10.5.3 (I guess) and up you can sync your google contacts with your iPhone.
Go to Address Book -> Preferences -> Accounts (tab) -> Synchronize with Google.
Synchronization will happened when you connect your iPhone to your mac and itunes starts backing up your iPhone. It will grab contacts and if there are some conflicts it will let you know.
BTW it works in both ways – now you can just modify your Address Book in mac and changes will occur in iPhone same as in gmail.