I can’t send an email through my iPhone or Mac

For iPhone:--> If you can’t send email on your iPhone, iPad, or iPod touch - Apple Support

For Mac:--> If you can’t send emails in Mail on Mac – Apple Support (UK)

If the issue persists, contact Apple Support directly, using this link:--> Get Support

Regards

Giulio

Talking about Apple Mail app on your iPhone, I suggest that you try deleting that gmail account in Settings>Mail>Accounts, then restart your device and re-add that email address to Apple Mail app.

Regards

Giulio

What did Apple support say when you contacted them? What suggestions did you get?