cheltzel for nationals of 80 countries visas are no longer required. See
http://www.flyertalk.com/forum/qatar...zens-80-2.html for more details. If your nationality is not mentioned then you may be able to use
http://www.flyertalk.com/forum/qatar...hour-visa.html or
http://www.flyertalk.com/forum/qatar...er-2017-a.html.
But in the case of the free city tour I am sure that visas would only be issued if required for the duration of the tour. For visa free citizens however I am also sure that they will be registered for the tour and be subject to the same restrictions as previously (not leaving the tour before reaching the departure areas of the airport again etc.).