As far as I can remember Google services required a Gmail account before Google+. Here is Schmidt commenting about the release of Google+ as an identity service.
My point is that Google's had a unified login system across all products (except legacy Blogger and YouTube accounts) since at least 2008. It doesn't really make sense to call it a GMail account - GMail was one of the services that it gave you access to, but it also was your account for AdWords, Analytics, Google Voice, Blogger, Reader, Docs, etc. You can find references to it on the web from 2009 onwards, as well as reports that the Chinese hacked it in 2010:
The "G+ is just an identity layer on top of Google" was a common soundbite reported by executives at the time, and perhaps they even believed it, but my engineers-level view as someone who worked with the G+ codebase was that this was mostly marketing speak to differentiate us from Facebook and deflect criticism of "Well, if G+ is a new social network, why would I use it over Facebook?" By saying it's the identity layer for Google, they can then say "Well, you use Google already, this is just the social layer that ties all Google products together." On a technical level a "G+ account" was just a bit set in your Gaia record that indicated whether you had opted in to G+ features.
http://www.forbes.com/sites/kashmirhill/2011/08/29/googles-e...