Well, we know they get an email address and, quite probably, a username associated with an account. Of course, that would only be of value to someone doing something sketchy...

Alternatively, he gets a large sample size of unwanted emails with unsubscribe links. This would be useful for someone writing a gmail plugin (or Thunderbird plugin, or new mail service, or whatever) designed to deal with unwanted mail that's not fully spam (i.e. complies with CAN-SPAM) but which is not desired by the recipient.