Thanks for taking the time to provide some additional details for a matter that must be highly irritating. I'm still scratching my head about
it, but, no more than you, I guess.
I agree with Apple that updating your OS shouldn't have had anything to do with your server-based Gmail. Those two things are independent.
Thus, the only possible explanation I could imagine was your using a third-party e-mail client (e.g. Outlook, Thunderbird, etc.) to download your Gmail to your iMac and have the server-based copy deleted.
But, that's not what happened. Plus, as previously noted, even if your Gmail messages had been downloaded locally (to your iMac), an OS upgrade still shouldn't have deleted any messages.
If an App or another person would have had access to your Gmail account/credentials, then I could understand the potential for something to happen. But, e-mails shouldn't vanish on their own. Air quotes on "shouldn't."
Finally, anything that would have been deleted in Gmail (via browser access) should have gone into your "Trash" folder. They would sit there for 30 days, before being automatically deleted, unless someone/something emptied the "Trash" folder sooner.
The only other thing I could imagine is this:
https://support.google.com/mail/contact/missingemailsSorry for bugging you about
this. In addition to enjoying solving problems, my many years of technical support (professionally and for family/friends) makes it nearly impossible to not help. It's a curse. Now, if I could just get an interview and get hired, now that I've been looking to go back to work. The time off has driven me a bit nuts.
Okay, I think I'm going to call it an "early" night and go to bed.
Nighty night.