Type Gmail in the Description field, otherwise the account will be labelled Exchange instead.

Tap Next at the top of the screen and then enter m.google.com when the Server field appears. Tap Next again and your account details will be verified. If they're all Ok, you'll see ticks next to each field.

On the next screen you can choose to enable or disable Mail, Contacts and Calendars for the account. Tap Next and the account will be added.

Finally, tap on the new account in the list so you can choose how many days' email to synch and which folders to update via push communication. Most people should leave this set to Inbox.