If you want to use Google Mobile Sync to keep your iPhone synchronised with your Google Calendar and Contacts, you’ll have to backup any existing contacts on your iPhone first, otherwise Google Sync will delete them.
Fortunately, there is an easy way to do this and you can use iTunes to upload your contacts to your Google Contacts list. You can then set up Google Mobile Sync and sync those same contacts (along with any others) back to your iPhone. Here’s how.
Connect your iPhone to your computer and, if necessary, fire up iTunes.
Once iTunes has recognised your iPhone, click the iPhone under Devices in the left pane, then click the Info tab in the right pane. Under Contacts, tick the Sync Address Books contacts option, then tick the Sync Google Contacts option – a Google agreement will pop up and you’ll need to click the Agree button to continue.
You’ll be prompted to enter your Google ID – the email address and password you use to access the Gmail/Googlemail account whose contacts you want to sync. Click OK when you’ve done this.
Your iPhone and Google Contacts list will then be synchronised. If you’re prompted to replace or merge your contacts on the iPhone, select the Merge Info button.
Don’t worry about the contacts that get transferred to your iPhone, since they’ll get deleted by Google Mobile Sync. It is worth spending a little time sorting out your Google Contacts though, since Google Mobile Sync only synchronises the My Contacts* group.
* At least according to Google – in practice, the All Contacts group gets synchronised, so this could be a bug in the service or a mistake in Google’s documentation…