If none of these options work, try to use aeroplane mode. This will stop all your network connections and will solve your issue. You can also try disabling background app refresh. This will also fix your Gmail problems. Check other folders in your Gmail app. If you are not able to download new emails from your Gmail account, you may experience this problem. This will prevent your email client from receiving new messages.
See also - https://www.fuzia.com/article_detail/321005/what-to-do-for-gmail-wont-load-mails