Is it permitted to run a gas pipe through the now unused chimney to get to the attic and from there to the other side of the house?
In your title, you say "unused"; then...


  My furnace and the gas pipe that feeds this is in the basement. I
  would like to rung the gas pipe through that furnace...


So which is it? The furnace chimney that you are using for the furnace now, or an old furnace chimney that is now unused? Because if it is an old unused chimney, that can now become a "chase" from the basement to the attic and is perfectly legit. But if it is the chimney FOR your existing furnace, then no, you cannot use that for anything else.