If you are experiencing some issues with the Gmail sync setting, then maybe this issue is occurring because you are not connected to the internet. In order to use the Gmail sync setting, you need to have a working internet connection. So, you must check whether your internet is working properly or not. If the internet is working properly, then check sync settings. In order to do so, you need to launch the Gmail app and then go to the Menu. Now, select Settings. In case you are using multiple accounts, then select the account in which the email syncing isn’t working. Now, check the option of “Sync Gmail” if it is unchecked.