Why do military pilots report "gear down" during their traffic calls?
1In the U.S., for Military aircraft, ATC is required to: 


  "Remind aircraft to check wheels down on each
  approach unless the pilot has previously reported
  wheels down for that approach". [ref: [FAA Order 7110.65W], para. 2-1-24]


Normally, the phraseology suggested in your question would reflect the pilot's response to this ATC reminder. (For example: "[call sign] Check Wheels Down, Cleared for [whatever]")