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I just turned on google sync in address book on mac, then edited what I wanted and synced it via itunes to my phone. This way I don't get the battery life hit normally involved.
 
I was just trying to figure this out and realized the easiest way was to enter iTunes -> iPhone -> Info Tab -> Check Sync Contacts with Google Contacts
Enter your Gmail account info. A message box will appear with Merge or Replace. I just clicked on Merge.

Hit Apply and all my contacts from my iPhone was synced to Gmail.
 
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