Try the following steps in order:
1. Check the internet connection of your iPad. If its not connected to the internet, please enable the internet connection using your Setting on your iPad;
2. Check whether Gmail is working properly from an Gmail web service or application.
3. Re-Authorize the Gmail Tag, by pushing “Reauthorize Gmail” after activating the dialogue by tapping the Gmail Tag icon on your tag list.
4. Activate a dialogue by tapping the Gmail Tag icon on your tag list, and push “Forget this Tag” once and delete Gmail Tag. Then, re-add the tag back on your tag list.