Hi Wondering_1,
Additional information would be necessary to offer more targeted suggestions for your concern.
When you say "on gmail", if you're referring to the Gmail app, it may be necessary to consult with the developer of that software to troubleshoot app behavior or performance. The article How to contact an app developer outlines how to get in touch with the correct group.
If you are instead using the Mail app with a Gmail address, then the information at If you can’t send email on your iPhone or iPad should be your next stop. Work through that article, consulting your email provider if necessary, up to removing and re-adding the account to the device per the included directions.
Regards.