1. Close the mail app on your ipad
2. Go your Mail Settings on from Google’s web mail client (It’s under the Forwarding and POP/IMAP tab) and make sure IMAP is enabled. If it’s not, enable and save. If it is, disable, save, re-enable, and save again.
3. Relaunch you mail app. This worked for me.
If it doesn’t work, the next step is deleting the account on you iPad, and adding it again.