See https://bugs.kde.org/show_bug.cgi?id=404990 .
In short, it's not a bug in kmail or akonadi, but rather a "political" problem because Google themselves blocked access. Doesn't mean it shouldn't get fixed from the KDE side though, rather the opposite. But me personally don't really know what the remaining problems are. Maybe somebody else here can step up and finally help to solve it? Kind Regards, Wolfgang