How to Set Up Gmail on an iPhone

Go tho this link provided below

http://www.wikihow.com/Set-Up-Gmail-on-an-iPhone

you can also try these if it asks

Incoming settings, IMAP server: imap.gmail.com, Port: 993, Security type: SSL (always)

Outgoing settings, SMTP server: smtp.gmail.com, Port: 465, Security type: SSL (always)