In the last few months several sources have reported that Google is using your Gmail to train its AI. Google denies it. Or they may have re-enabled it after pausing for a while.
Regardless, I thought I’d post a link to Appleinsider, where they explain how to turn off the AI training options, which are set up as “opt out” instead of “opt in.” (There ought to be a law against having to opt out of optional things.)
I floundered through it, with some screens not looking quite like what the article describes, but I think I found and turned off the two relevant permissions that had been checked. Shortly thereafter, an option appeared at the top of my Gmail page to opt in to something-or-other. I ignored it.
Of course those could all be dummy settings and Google is still doing whatever it wants to do. We have no way of knowing. It could be like the dummy thermostat at the office that everyone thinks they are controlling.

(I had to laugh at the caption that came with this particular image.)
Another explanation can be found on the Malwarebytes website, where the AI training story originated, and you may find their instructions easier to follow.
Do with this information what you will. I know that in my 25 years’ worth of emails, I have lots of documents and correspondence (contracts, receipts, bank statements, medical reports, etc.) that are decidedly private, not fodder for Google’s AI.
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Addendum:
In what may or may not be a coincidence since I changed the above settings yesterday, this started appearing on some of my emails today:

Not sure yet what to make of this.

I hope they don’t train on my email — that will screw up the world for decades.
LOL!
ooh, I started getting that too and I wondered why
Maybe it’s something new that Google is doing now for everyone?
not sure, I though it was something I’d done
I thought maybe it was because I made those settings changes I mentioned above.
Thanks for this info!
It may or may not be true, but why take a chance (unless it eliminates some feature you value).
I am not, she said loftily, a Gmail person.
But then, she was forced to add, Outlook is in all probability just as much a training tool. Hmmmm … (thoughtfully) MY emails might well be utilized for their length – and coverage of topics.
[chorus of agreement from reluctant MR email recipients]
AI probably trains on anything it can find on the internet, although prompters may direct it to specific targets. Who knows. No doubt your scintillating prose is much in demand.