Early and pre-war Japanese types rarely had given names.
The practice started by the IJN in late '42 while the IJA used some given names before and after that time, raptor names were most popular for their fighters.
Guntei means "army" or "troops" in Japanese, it makes no sense for a "name".
Unless someone mis-translated a document that referred to the KI-51 as an "Army" type (which it is, of course) and thought it was the type's name.
All I can tell you is what I read in the book. It is a Japanese publication from 1956. It says "Shitei" was strategic reconnaissance "Guntei" was tactical recon. The tables in the book lists Ki-51 Guntei with it's purpose as "army reconnaissance". You may see their sources in the Thread "Nakjima Homare - What's the verdict" posts 33 & 34. They did not use footnotes as per western publications but took the info from interviews with those listed. Many of the names are well known.
Just a brief follow up on Japanese names for their planes. The "Airview" book only has a name for many of the army's planes. The Ki-100,Ki-36, Ki-57,Ki-48, and Ki-21 are some which are indicated no Japanese name was given.
The Ki-51 may have been called Guntei much the way we call the C-47/DC-3 Gooney Bird. Unofficial.
九九式襲撃機(九九襲/九九襲撃)
Kyu-kyu-shiki-syugekiki (Kyu-kyu-syu or Kyu-kyu-Syugeki for short in the field)
meaning Type 99 Assault Plane
派生型
Variant as same Ki-51
九九式軍偵察機(九九式軍偵/九九軍偵)
Kyu-kyu-shiki-ghunteisatsuki (Kyu-kyu-shiki-ghun-tei or Kyu-kyu-ghun-tei for short in the field)
meaning Type 99 Reconnaissance Plane
九九式襲撃機(九九襲/九九襲撃)
Kyu-kyu-shiki-syugekiki (Kyu-kyu-syu or Kyu-kyu-Syugeki for short in the field)
meaning Type 99 Assault Plane
派生型
Variant as same Ki-51
九九式軍偵察機(九九式軍偵/九九軍偵)
Kyu-kyu-shiki-ghunteisatsuki (Kyu-kyu-shiki-ghun-tei or Kyu-kyu-ghun-tei for short in the field)
meaning Type 99 Reconnaissance Plane