I have upgraded from 2.3+ to the most recent 2.4.5. I am in the middle of adding back plugins, but I do have Google Apps working. At first, I saw the behavior you describe where the user is redirected to the "You are already logged in as..." page. I looked at the auth plugin settings, and Gsaml had the eye closed. After I opened it, the SSO worked.
As far as user sync, that is something that has been inconsistent for me since upgrading from 1.9. I do not use the GData cron for user sync. It has deleted users that were not supposed to be deleted in the past. I have events enabled in the Google User Sync block settings. In 1.9, this worked flawlessly. Users created or deleted in Moodle were created/deleted in Google at the same time. The user creation has not worked for me in Moodle 2.x without the code change mentioned here. That change creates users in 2.4, but in my initial testing, users do not seem to be deleted in Google if deleted in Moodle.
I will post back if I find anything else of value.