Location of past IPs used to access GMail

Knowledge about items on the inbox/address book

Location of devices used to access the account

Knowledge of past passwords

Not sending password reset emails to secondary emails that have just been added

If a human looks it would be trivial for me to do the legwork to prove I own the google account.

I've had more than one job that uses gmail in the office, including my current one. My boss's account is presumably authenticated and if I bugged him he would vouch for my identity.

I have correspondence with a bunch of people in my google account going back years. I could bug any number of them to vouch for me.

I've had, in the past, a few work accounts that used google, that mad my picture associated with it. I can do a google hangout to show that that is still my face.

I have a driver's license with my real name on it, which matches my google account.

I control the phone number associated with my google account.

. . . A hacker could compromise one or two of those, but it would be hard for him to get a majority of them, even if he had my phone and email in his control.