What is the use of "pyro" in the naming of organic compound?
So, as @long and @permeakra stated, the name pyrogallol came from the fact that it is obtained by heating gallic acid. The IUPAC goldbook clearly states that:- 


  "pyro" is used as a prefix designating compounds formed by heating a compound,
  usually with the elimination of water, carbon dioxide, or other simple
  molecule, e.g. pyroglutamic acid from glutamic acid.


So as per rule, it is pyrogallol and not gallol.