Why gmail not working on iphone want to fix this issues now you can search this over internet you will see some brief steps as follow :

Solution 1 > Check if you have connected to the Internet
Solution 2 > Log in another Gmail account.
Solution 3 > Remove Your Gmail account from your devices
Solution 4 > Re-verify device's client's access via web
Solution 5 > Check The Gmail Website For Alerts
Solution 6 > Double Check That IMAP Is Enabled

Either you can check for full information on the given website either you can contact mention there helpline number
http://www.livehuntz.com/gmail-not-working-on-iphone-how-to-fix-it