You can use Google Sync to sync your Mail, Contacts, and Calendars to your iPhone. Select which ones you'd like to sync on this page.
If you choose to sync your contacts and calendars, you'll see a message pop up. It asks: "What would you like to do with existing local contacts on your iPhone."
To avoid deleting your existing contacts, make sure you select "Keep on My iPhone."
You'll see a warning that you may see duplicate contacts. But, again, if you want to avoid deleting all of your contacts, this is your only option.