The trick is the same as logging into Facebook using your Google account or to Google+ using your Facebook account.
The first time a user visits your site even if they are logged in to another Google product they are asked to allow this site to allow them to login using their Google account. If they accept then in their Google account a note is made that this site can now be logged into using their Google account.
The next time the user visits the site they will be asked to login to their Google account, unless they are already logged in for instance via Google+ or Gmail. In that last case the Google account server will check the account see that the site has already been approved by the user and thus log them in without showing the screen asking them to allow the site.
So for a user to use a different login simply have them logout. Once done send them to the login page again and Google will popup with the same question with of course a login box. After all they just logged out of their Google account...
So for you to be able to offer a switch user option you will have to ask the users to logout and log back in with a different account or make it so that your application sends the to the logout page and as soon as they return back to the login page again. At the moment there is no switch user option offered by Google so the ugly load logout page, load login page is the only option if you want to use the Google authentication.
If you need to offer your users this switch user option you will have to simply create your own login service at which point you should be able to switch the user without all the page reloading in the middle.