Syncing Google Contacts
 Sync your Address Book.
- Open Address Book.
- Click on Address Book > Preferences
- Click the checkbox for Sychronizing with Google
- Enter your username and password.
 Add a Google Account to your iPhone
- Go to Settings > Mail Contacts Calendars > Add Account...
- Select Gmail
- Enter the appropriate username and password.
- Set the sync options as appropriate: Mail, Contacts, Calendars will all be things you'll likely want On.
- When you turn Contacts On, it will ask you what to do with your phone's current contacts. When it asks to delete the phone's local contacts, you should agree to this step PROVIDED that all your contacts information is on Google's server.
Now, your iPhone should contain all your Google contacts.
A related but different issue involves having duplicate contacts on your phone. I had this for a while, and it was pretty annoying. My profile pictures for various contacts were changed, but not really (depending on which of the two contacts I had altered), and the custom ringtone assignments sometimes worked.
For those with similar problems, the solution is easy.
Removing Duplicate Contacts on the iPhone
 Go to Settings > Mail Contacts Calendars
 Select your Gmail account.
 Go to Contacts and turn it Off. When it warns you of the contacts' deletions, accept the action.
 Now go back and turn Contacts On. When it asks you what to do with the local contacts, choose the actions that will delete them.
Now, your duplicates should be gone! And the world was right again.
Thanks to ElectroDisco who posted on this Google Forum thread with the above answer.
For those who are interested, Google's official documentation on the matter is here.
Google's official documentation about contact syncing can be found here.