Gmail stopped working in the Mail app on my iPhone.

Remove your email account and set it up again

Make sure that all of your emails are there or make sure that your email is saved somewhere other than your device. On your device, go to Settings > Mail, then tap Accounts. Tap the email account that you want to remove. Tap Delete Account.

If you can't receive email on your iPhone, iPad, or iPod touch ...

If you can't send an email on your iPhone, iPad or iPod touch

1. When you make an iOS or iPadOS backup in iCloud or iTunes, it backs up your mail settings, but not your email. ... 
2. Make sure your device is connected to the Internet.
3. Check with your email service provider to find out if there's a service outage.

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If you can't send an email on your iPhone, iPad or iPod touch ...

Kindly confirm that you are using the latest version of the Mail application, and check for any applications which you installed last time are creating any issues with the other applications, if the issue persists try to remove the application from the device and try installing a new from the app store.

If still the issue persists check the Gmail that whether you have made any restrictions, this might also cause this issue.