[garibaldi]

Here is the solution:
I had to change the "Mail display language" in Gmail settings back to original when the account was created!!!
My account was created using Greek as language and I have changed it to English.
After turning back to Greek, [Gmail] system labels/folders reappear to Zimbra desktop!
This problem does not appear on other email clients (Outlook, Thunderbird).
Perhaps it is related to "login options" in Options/global/language of Zimbra Desktop..??