You can access Google Docs without having a Gmail account (see video).  Also, here is a way to combine "access" rights for multiple email accounts into one Google Services account - that way when someone sends you a Google Doc invite to [email protected] or [email protected] - you can access the Google Doc with either email address.

Basically it all boils down to - Gmail is seperate from your Google Services account. They can be the same - but they don't have to be. Your Google Services account is the one you use to access all of Google's services. And it can be based on any email - or multiple emails - and none of them have to be Gmails.