I never used kmail(2) before this and was quite surprised when after everything was configured, absolutely nothing is shown where the emails are supposed to be there.
The Gmail IMAP account is shown "Ready" both in kmail's and Akonadi's configuration systems.
Is there any additional step I need to take before the mails are shown ?

(I'd gladly provide some additional details, but I don't know what'd help here.)

Do you see emails listed, or nothing at all? Do the email show up in the web UI?

einar wrote:Do you see emails listed, or nothing at all?

For "Inbox", nothing at all.
Some sent items and notifications from over an year ago are visible in some other folders.
From that I'd guess that some syncing is not done... despite it saying that it is complete(at the bottom right of the Status Bar ) .

Do the email show up in the web UI?

Of course.

Any useful error messages in ~/.xsession-errors?