Actually, I was an early member to Gmail, and I have no memory of giving them any personal information. Since Google+ requires personal information, this conflicts with giving Gmail account holders G+ accounts if I continue to not provide personal information.
It sounds like this is only for people who create new Gmail users.
Also, I wonder if it will automatically happen if you make a public profile in Google.
That's the big thing: I think, when I last made an account, a public profile still wasn't the default, may not have been an option. I'm not really sure, but I know my secondary accounts are not tied at all.
You can probably disable the Google+ bonus by declining a public profile.
Mind you, the public profile thing is separate but the first line for your personal information. This is in the Google Account Settings, or whatever it's called.