Follow these steps on your iphone to setup gmail.
To add Gmail Add your Exchange account
Tap Settings > Mail, Contacts, Calendars > Add Account > Exchange.
Enter your information
Enter your email address and password. If you want to, you can also enter an account description.
Connect to the Exchange Server
Your iOS device will try to find your Exchange Server. You might have to enter additional server information. If you need help, contact your Exchange Server administrator.
Sync your content
You can sync Mail, Contacts, Calendars, Reminders, and Notes. When you're finished, tap Save.
Edit your Exchange settings
Tap Settings > Mail, Contacts, Calendars, select your Exchange account, and tap Account Info. You can also go here to set up automatic out-of-office replies and change how often you want Mail to sync (the default is 7 days).