Why IMAP gmail.com not working?

Are you able to send and receive emails?

Have you tried to remove the email account and re-add?

Try resetting your Mail Account.

First close your Mail App completely from the app switcher window by double clicking the Home button and swiping up the Mail preview pane up until it disappears from the screen. You may have to look for the Mail pane by scrawling to the left.

Then go to Settings > Mail, Contacts, Calendars > locate your email provider and click on the account. On the next page Delete Account.

Then do a forced restart. Hold down the Home and Wake/Sleep buttons at the same time for about 15-20 seconds until the Apple logo appears.

Then go back to Settings > Mail… and add the account again.

Awesome directions, but a force restart is only recommended "as a last resort, and only if it's not responding."

Yes I know, that has been mentioned here many times before and in the option of many Level 9s and 10s Apple should revise that article. A forced restart is standard procedure, I do it regularly when needed, never had a problem. Try it and see.