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The dates
The 20世紀日本人名事典 (Nijusseiki Nihon jinmei jiten), on Kotobank, has him born on the 11th day of the second month of Tenpō 7 (1836) in Ōmi, in the Inukami district, with the surname Tsukamoto, and dying on 11 July 1918 (Taishō 7). His age at death does not agree in the books cited by the reference service of the National Diet Library: 83 years for Honma Kōhaku and 84 for Nakamiya Keidō, counted in the manner of the time. The Metropolitan Museum gives 1834 as his year of birth. His signatures give his age: 72 in 1907, 76 in 1912, 78 in 1913 and 81 in 1915, according to the titles of the ColBase records. Counted in the Japanese manner, which gives one year at birth, those of 1907 and 1913 fit 1836; those of 1912 and 1915 are off by one year, one in each direction.
The Gassan family
According to Kotobank and the Japanese Wikipedia, at the age of seven he was adopted by the swordsmith Gassan Sadayoshi (月山貞吉), who at the end of Edo had moved from Dewa to Osaka, and with him he learned the craft. The Gassan of Osaka took their name from the old swordsmiths of Mount Gassan, in Dewa. The same Wikipedia follows the family: his son was Gassan Sadakatsu (月山貞勝, 1869-1943); his grandson, the second Gassan Sadakazu (1907-1995), a Living National Treasure from 1971, and his great-grandson, Gassan Sadatoshi (月山貞利), born in 1946.
The years without swords
The Nipponica encyclopaedia relates that with the ban on wearing swords (廃刀令, haitōrei) of 1876 he went through hard years. The 20世紀日本人名事典 (Nijusseiki Nihon jinmei jiten) says that he gave up forging and took it up again in 1885 (Meiji 18), although it dates the ban to Meiji 4 (1871), the year of the order that allowed people to stop wearing them. In those years, according to the reference service of the National Diet Library, which cites a biographical work of 1907, he forged a tachi and a tantō by order of the court in 1869 (Meiji 2), and for the 550th anniversary of Kusunoki Masashige he gathered more than two hundred swords and took charge of testing their cutting.
Ise and the imperial household
A katana of his from 1894 was shown at the 4th National Industrial Exhibition, in Kyoto (1895), won a second-class prize (有功二等賞, yūkō nitōshō) and was bought by the imperial household. The Tokyo National Museum holds a katana signed 大阪住月山貞一精錬之 (Ōsaka-jū Gassan Sadakazu seiren kore), dated December 1894 and with the carving also by his hand. On 4 April 1906 he was appointed artist of the imperial household (帝室技芸員, teishitsu gigeiin), at the age of 71; in the whole history of that title (1890-1947) there was only one other swordsmith, Miyamoto Kanenori, appointed on the same day. In 1912 he forged, by order of the Home Ministry, the offering tachi of the grand shrine of Ise.
In 1912 he gave a tantō to the Tokyo National Museum; as it stayed in the circle of the Emperor Taishō, in 1913 he forged another for the museum, according to its record in ColBase.
The command swords
The Japanese Wikipedia credits him with the design of the marshal’s sword (元帥刀, gensuitō), in the shape of the Kogarasu-maru, and with the swords of honour for the graduates of the Army War College. Another article of the same Wikipedia, the one on the marshal’s baton (元帥杖, gensuijō), gives the command sword of the Emperor Meiji (around 1906) to Gassan Sadakatsu and that of the Emperor Taishō (1915) to a “second Gassan Sadakazu, artist of the imperial household”. That does not fit the dates: the only artist of the imperial household in the family was he, and the second Gassan Sadakazu was born in 1907. And the same Wikipedia’s article on Miyamoto Kanenori (宮本包則) gives Kanenori that generalissimo’s sword (大元帥刀, daigensui-tō) of the Taishō enthronement: the same encyclopaedia attributes it to both.