That's you/OAuth giving the provider your google account id, which is your @gmail.com email.

I don't generally want any site to have anything they can use that associates me with other sites. If 2 sites get the same email for me or the same GAIA id, or the same anything then I won't use the id system. (with obvious exceptions - see below)

This includes "privacy first" companies like Apple and their Apple Pay system where I went to a restaurant in SF. The bill was a QR code that took me to Toast with the option to pay via Apple Pay. The apple prompts told me my email address would be shared and there was no option to say "no" so I bailed out and paid the waiter directly.

Sometimes I need my real name and address for shipping. In those cases that can't be helped. I also have to give my CC card for a purchase. But there are sites I want to sign up for for which I don't need to give that info. A "one click to sign up" option would be useful if I knew it was giving random data. An example might be medium.com or substack.com. They don't need my real name nor do they need my "real" email. If I was sure this "one click sign up" didn't share a common one I'd consider using it.

Maybe even better, if it was managed similar to subscriptions in iOS where I could trivial revoke any membership at will from a central location, with the understanding that there'd be no recovery since signing up again would get random new data and so no way to associate the new with the old.

Google uses GAIA for ID though, which is not the same as gmail address