The following set of steps produces a 403 error instead of granting access to the calendar:

In Zimbra webmail, Calendar > Right Click Calendar > Share Calendar > External guests (view only) > $Email and $Password > "Allow user(s) to see my private appointments." > Send Standard Invite > OK

$Email then gets the message from Zimbra with a URL like this:

I then modify it to add the username and password. Note that I've URL Encoded the @ so that whatever tries to GET the calendar doesn't try to get it from gmail:

I am told Google Calendar is unable to fetch the URL. After visiting the above I get a 403 error with the specific text "permission denied: you do not have sufficient permissions".

I would think that I should not get a permission denied. The closest thread I could find to this was the following:

However, they are trying to do it the other way, from Google into Zimbra. I'm trying to allow someone with Google to see my Zimbra calendar. Please let me know if anyone has seen this problem.