Google now allows people to be signed into multiple Google accounts at the same time. However, I don't think that Google accounts have their own unique OpenID identifier URLs (it's the same for everyone: https://www.google.com/accounts/o8/id).
This presents a bit of a problem. Let's say a user is logged in to two Google accounts: A and B. However he only has an account on my site for B. I have no real guarantee that when he clicks my OpenID link that it will authenticate with B instead of A... all I can do is see the result, and if it was A, give the user a confusing error message that says "You seem to be logged into Google Account A, please logout".
Is there a better solution?
Why don't you logout the user from all google accounts first and then take him to the login page ?