My gmail account not working

If you're using Apple Mail app, AND if that gmail account works fine on a computer, try this work around: delete that email account in Settings>Mail>Account, then restart your iPhone, then re-add the same gmail account, as per these instructions:--> Add an email account to your iPhone, iPad, or iPod touch

Of you're using Gmail app, contact Google Gmail support team and ask them.

Regards

Giulio