From Grand Sumo: The Living Sport and Tradition, rev. ed., by Lora Sharnoff (Weatherhill, 1993), pp. 178-179:
As of May 1992, there were forty-one gyoji, all with the assumed professional name of either Kimura or Shikimori. The gyoji’s role is believed to date back to the late eighth century, but apparently did not take on its present form until the late sixteenth century when Oda Nobunaga reigned as the most powerful military lord in Japan. The houses of Kimura and Shikimori came into being in 1726 and 1768.
For nearly two centuries there were clear distinctions between the two lines of gyoji. Even now some fine differences exist in the way they hold the gunbai, or war paddles, when calling out the contestants’ names. A Kimura keeps his palm down; a Shikimori has it up. Yet nowadays a referee can start out as Kimura, switch over to Shikimori, and go back again to Kimura as he moves up the referee ranks.
Similar to the way several apprentice sumotori perform under their real names, some of the young referees also use their own given name as part of their professional name (such as Kimura Hideki) in the early part of their career. A more old-fashioned sounding name, like Zennosuke or Kandayu, will be assumed by the time they have climbed high enough to officiate matches at the juryo level.
The highest ranking referee is always named Kimura Shonosuke and the second highest is always known as Shikimori Inosuke. Given the moving between the gyoji families nowadays, this means that the man assuming the name of Kimura Shonosuke was previously known as Shikimori Inosuke, and that he undoubtedly performed under at least one or two different names before that. The referees must work their way up through the ranks just ike the sumotori. The youngest gyoji, like the youngest rikishi, is likely to be fresh out of junior high school. He must be affiliated with one of the sumo stables and, just like the sumotori, is likely to live there until he gets married.
A separate stable once existed for the referees but it was closed in 1973. (However, they still have their own large dressing room inside the Kokugikan.) Now almost all the sumo stables except for some of the newest or smallest, have one or more gyoji attached to them.
Again similar to the sumotori, the gyoji’s promotion through the ranks is based primarily on ability, though seniority can play a small part in the referee’s case. Nevertheless, just as an exceptional sumotori like Kitanoumi and Taiho can become yokozuna at age twenty-one, the previous Kimura Shonosuke XXVII—the “grand champion” among referees—was promoted to the position at the extraordinarily early age of fifty-two. For some years, Shonosuke XXVII had a second-hand man, Shikimori Inosuke XXIV, who was about six years his senior. Inosuke XXIV reached the mandatory retirement age of sixty-five at the end of 1983; Inosuke XXV was a bit younger and became Shonosuke XXVIII in 1991.