If you set your Email for automatic syncing then, it will keep your emails and attached documents safe. So, it is advisable to set the sync for automatic start. For that, open the Gmail app and then tap the three-bar menu icon. After that, tape the Settings and then tap your account name. In the Data usage heading, tap on Sync Gmail. Be sure that you are online to sync your Gmail app that required either a cellular data connection or Wi-Fi.