Strangely, it's only on mobile devices.
Even stranger, on the same device, the gmail web interface works perfectly.
Strangely again, google forum doesn't give consistent answers when upset users ask them. No solution to fix it... Well not exactly. They answer that they should download the gmail app from the Appstore and use it instead of the iOS mail client.
Oh yes another little thing: when you quit the google help forum, google redirects you straight away on the google gmail app download page.
Ok let's face it, this looks like google plays rogue marketing to promote his gmail app over iOS native mail app.