1. Launch Gmail and connect your iPhone to the computer. Transfer your iPhone contacts to Gmail in the form of vCard file.
2. Go to "Settings" on your iPhone and tap on "Mails, Contacts, Calendars".
3. Select "Add account" and tap on "Exchange" afterwards.
4. Enter your full Google Account email address, username and password in the interface.
5. Click "Accept" in the pop-up window and you'll get an interface shown right below.
6. Input "m.google.com" in the "Server" box.
7. Select the services you'd like to sync. Turn on contacts so you can sync Gmail contacts to your device now. Select the "Keep on my iPhone" option when prompted if you would like to keep your existing contacts on your device. That will allow you to keep duplicate contacts on your device.
8. Return to "Mails, Contacts, Calendars" and select "Contacts" and you'll find "Groups" newly displayed on the top left corner.
9. Press "Groups" option to get the window below. "All Contacts" are the ones on your Gmail and iPhone contacts.
If you still not clear, please read the below article: