My gmail accounts are accessed simultaneously. When I log in sometimes, I am logged off because "another user" has logged on. I have two way authentication turned on. Gmail has a "Recent Activity" section that details the last 10 IP addresses used to access the account. They only show mine. Is it possible to manipulate Google's reporting of the IP addresses accessing the account by hijacking my IP address and simultaneously intercept the two-factor authentication code that comes to my cellphone? Or if they are on the same network?
I would think that this would be hard, if not impossible to spoof. For a TCP session to be established the three-way handshake needs to occur, and I'm not aware of a method where a spoofed IP address is able to successfully complete this.
If the traffic claims to originate from your IP address then it's almost certainly coming from your IP address. If you didn't have 2FA enabled I'd be concerned that perhaps another computer in your network was compromised, but I would wager that there isn't anything malicious going on.
I've never experienced this error myself and I have two factor authentication enabled, so I'd say reaching out to Google to inquire is your best course of action.