Accessing the contacts that you store with the most popular email service providers -- including Google -- requires just a few simple mail account setting tweaks on your iPhone. In fact, you can sync all of your contacts from multiple sources with your iPhone and unify any resulting duplicate contact records straightforwardly. But, if Gmail is the most important of them all, then it's important that you also make it your default account. This will ensure that your Gmail captures all new contact information you input directly onto your device going forward.
Tap "Settings" on the Home screen. Next, touch "Mail, Contacts, Calendars."
Tap "Add Account," followed by "Google" on the Add Account screen. If you've already added your Gmail account to the iPhone, skip ahead to Step 4.
Type your email address and password in the fields that request them on the Gmail screen. Optionally, type in an account name as well, and replace the automatically generated description with a more memorable or specific one – for example, "Work Contacts," "Builders," or "Family and Friends," then tap "Next."
Toggle the "Contacts" switch to the "On" position. The switch should appear green. Touch "Save."
Tap "Contacts" on your Home screen. Your Gmail contacts automatically sync to the iPhone.
On an iPhone running iOS 5 or 6, do this instead for Step 2: Choose “Other,” followed by “Add CardDAV Account” and then “Google.”
When adding a Google account with two-step verification enabled, the iPhone will ask you to enter a special passcode that you’ll receive in the manner you specified -- via text, voice call or Google’s mobile app.
New contacts that you create on your iPhone are added only to your default account. If you have other sets of contacts synced to your iPhone besides Google – such as from Yahoo, AOL, iCloud or Outlook accounts – make the most important among them your default. Tap “Settings,” followed by “Mail, Contacts, Calendars.” Scroll down to the Contacts section, touch “Default Account” and select your default.
You may end up with duplicates in your Contacts app after syncing Gmail contacts to your iPhone, especially if other sets of contacts are also being synced to your device. To alleviate this, create a unified record for duplicate contacts. Open the first record of a contact, followed by “Edit.” Then, tap “Link Contact” at the bottom of the screen. Find and tap the duplicate record in your list of contacts, followed by “Link” and then “Done.” Note that if you make changes on either record, the changes sync only with its source. For example, if you change the phone number of a Gmail contact, the information is updated on Google only and not on the contact’s record that comes from iCloud.
Information in this article applies to iPhones running iOS 7. It may vary slightly or significantly with other versions or products.