The recent fast of Gedaliah makes me wonder about the origin of the fast.  The first textual reference is in the 7th chapter of Zechariah.  The exiles (now returned from the Babylonian exile and now living in around Jerusalem) approach the Kohanim and the prophets of the day (about 520  BCE) and ask if they should continue to fast in the 5th month.  He replies somewhat cynically- for whom have you been fasting these 70 years( of the exile) in the 5th and the 7th month.  Finally in the 8th chapter he says the fast of the 4th, 5th ,7th and 10th month will be days of joy- Just love peace and truth.  So it appears that the fast originates in the exilic period.   But you have to ask yourself about the fast of the 7th month.  Obviously not referring to Yom Kippur( that is a Torah law).  Look at Nehemiah chapter 9.  The exiles about 50 to 70 yrs later fast on the 24th of the seventh month.  To me that seems to have been an ad hoc fast due to their realization that their observance of the Torah had been less than perfect.
   So again I ask can the origin of the fast be related to the assassination of Gedaliah as related in Melachim 25 and Yirmiahu 40.  Gedaliah was a Babylonian appointee who probably took office in the month of Av 586 BCE.  He was assassinated probably 6 weeks later.  ( there are some who question this and say that he was not killed until a year or two later.  The context to me indicates a rapid series of events- I think 6 weeks is a safe bet).. In any event whether 6 weeks or two years- ask yourself is there any reason the exiles 70 years later would still be fasting for a Babylonian functionary who died before most of them were born.  I cannot think of a good reason.  By the time of the Talmud the fast of the 7th month is clearly associated with Gedaliah.  The Talmud says the death of a Tzaddik is equivalent to the burning pf the Temple. 
I think Chazal associated this fast with Gedaliah for a simple reason.  You cannot have a fast without a reason.  People get too hungry to fast without a reason.  Secondly- if you check your Tanach disk -searchword- seventh month- there is only one mention in all of Tanach that could in any way be associated with the destruction of the Temple, and that would be the death of Gedaliah(which by the way has no specific date mentioned).
    Lastly, if the fast didn't originate with Gedaliah, how did it  start?  I have no idea but the Jews in exile certainly came under the influence of the Babylonians.  The Babylonians has many ideas about days and months which were good omen days and bad omen days.  So at present the origins of the fast remains ,in my opinion, speculative.