1. Open the Settings application on your device's home screen.
2. Open Mail, Contacts, Calendars.
3. Press Add Account....
4. Select Microsoft Exchange. iOS 4.0+ allows multiple Exchange accounts. However, if you're on a device that doesn't let you add a second account, you could also use IMAP to sync Gmail, CalDAV to sync Google Calendar, and CardDAV to sync contacts.
Enter Account Info
5. In the Email field, enter your full Google Account email address. If you use an @googlemail.com address, you may see an "Unable to verify certificate" warning when you proceed to the next step.
6. Leave the Domain field blank.
7. Enter your full Google Account email address as the Username.
8. Enter your Google Account password as the Password.
9. Tap Next at the top of your screen.
9a. Choose Cancel if the Unable to Verify Certificate dialog appears.
10. When the new Server field appears, enter m.google.com.
To access m.google.com, set the language to English (US).
11. Press Next at the top of your screen again.
Enable Mail, Calendar, and Contacts
12. Select the Google services (Mail, Calendar, and Contacts) you want to sync. To receive and respond to meeting requests on your device, both Mail and Calendar need to be turned on, and New events needs to be enabled in your Google Calendar settings.
To enable 'New events', sign in to your Google Calendar using the web browser on your phone or computer. Go to Calendar Settings > Calendars > Click on the Notifications for the calendar you want to sync. Under Email check 'New events' (and any of the other Invitation settings you want enabled e.g. changed events, cancelled events, event responses), and click Save.
13. There are three options for keeping your contacts synced. You can keep your device's contacts and sync just your My Contacts group, you can sync only Google Contacts and wipe your device's contacts, or you can merge your contacts via iTunes and sync only the contacts that stream with your phone.
13a. If you would like to keep your existing contacts on your device select the Keep on my iPhone (or iPad or iPod touch) option when prompted. This will also allow you to keep syncing with your computer via iTunes. If you choose to keep existing contacts, you will sync the contents of the My Contacts group.
13b. If you have all your contacts in Google and you want to start syncing them to your device and don't want to keep the contacts on your phone, tap Delete Existing Contacts. If there are no contacts on your phone, the contents of your All Contacts group will be synced.
13c. If you have all your contacts on your device and want to import them into Google, you can synchronize your Google Contacts with iTunes. All Contacts and Calendars that you sync with iTunes will be put into the On My iPhone Contacts and Calendar groups on your phone. iOS currently doesn't allow you to move Contacts or Calendar events from the On My iPhone groups to the Google Sync groups.
You've set up Google Sync!