It's not gmail accounts, but "services" (?) hosted on Google's cloud. Basically I see X.X.X.X.bc.googleusercontent.com addresses in the "Received" header fields, e.g. "22.185.141.34.bc.googleusercontent.com"

When doing a WHOIS on that IP we'll get a contact address for abuse reports: "[email protected]", but sending anything there, returns an error that the user doesn't exists.