Use the Users API. https://developers.google.com/appengine/docs/java/users/overview
UserService userService = UserServiceFactory.getUserService();
Generate a logout link via:
thisUrl = req.getRequestURI();
String logoutURL = userService.createLogoutURL(thisURL);
When the user clicks on the logoutURL, Google will log them out of the application. It will also log you out of all of your other Google sessions such as GMail, Drive, Calendar etc.
Geoff