Gmail fails to sync for various reasons, and a failure to sync creates many other issues. You may get messages like Gmail waiting for sync, Gmail getting your messages, Gmail getting your messages waiting for sync. In this case, the user will not have all of their messages, they may fail to send email, they will even receive an “account not synced” error message, or the app will just become slow.
If your Gmail isn’t syncing automatically, you will get the following errors:
· You will not able to send mail, or your mail is stuck in send.
· User will not be able to receive or load new mail.
· User can’t open or read mail.
· Inbox is not updating
· “Account not synced” error.
· Not receiving notifications for new messages
· App will be working very slowly.
Source: Gmail Waiting For sync