Checking around internally to see if this special user account has an ID associated with it.
Could you share with our team what you are trying to accomplish? Just in case getting an ID is not possible, maybe we have a workaround for what you are trying to do.
Off the top of my head, I'm not sure if this should work or not, but I would try making the GET request to the same url, but omitting the filter. You will get a list of all of the users and you might see the Guest user in there. If you do, then maybe the filter just doesn't work for guest. If you don't, then we would have to look into why it's not returning the guest user.