To configure the 'Mail' app on an Apple device for access to Gmail, just follow these steps:
- Make sure you've enabled IMAP in your main Gmail settings
- Open the Settings app on your device
- Tap Mail, Contacts, Calendars
- Tap Add Account...
- Tap Gmail
- Enter your account information, being sure to use your full Gmail address, including '@yourdomain.com'
- Tap Next
- Tap Save
And you're done. You can add additional Gmail accounts by repeating the same steps above.