Mount Gassan rises among the Dewa Sanzan, the three sacred peaks of northern Japan bound to the mountain asceticism of Shugendō, and the swordsmiths who took the mountain's name worked in its shadow in Ōshū, Dewa Province. The record that smiths signing "Gassan" are noted in the Kanchiin-bon Meizukushi as early as the period, and the lineage's oldest surviving signed blade is a simply signed Gassan, kept at the Gassan Shrine and judged to date from the late to early period. Around that early moment the corpus places Gassan Nagamitsu, whose signed answers to the "Ushū Gassan group, around the Ryakuō era" recorded in the , and Shōgun, a hand sometimes identified with Gunshō, dated Genchū 3 (1386). A later branch is carried by Gassan Chikanori, working into the mid and late (an dated Eishō 9, 1512). The line was revived in Osaka in the closing Tokugawa and Meiji years by Gassan Sadayoshi and his adopted son Gassan Sadakazu, who endured the Haitōrei edict and was appointed Teishitsu Gigei-in (Artist to the Imperial Household) in Meiji 39; his own adopted son Gassan Sadaichi continued the title into Taishō.
The forging is the school's signature. The describe , the grain undulating in a cedar-wave pattern with vortex-like gathering in the hollows of the waves, the combination the records call "Gassan-." Over the base the grain flows () and tends to stand; adheres and enter, and the northern steel takes a dark, metal-like (kana-iro) hue read as Ōshū temperament. The temper of the old line is a whose is (subdued), at times mixed with and and softened toward , the turning back in . Departures within the corpus mark the lineage's range: Chikanori's late work can drop the entirely and absorb and manners, while the Osaka revival smiths command with thick , flamboyant -laden , , and in a full idiom, and both Sadakazu and Sadaichi added meticulous , from Bishamonten to taki Fudō and dragon carvings carved by the hand that forged the blade.
For recognition the point first to the wave-grain and the subdued, often slightly northern , and where these recede, to the dark steel and standing grain that still betray an Ōshū hand. Signed examples of the old line are described as extremely rare, which raises the documentary weight of Nagamitsu's and Shōgun's dated . Provenance threads through the school's own sanctuary: the founding and an Important Art Object both reside at the Dewa Sanzan and Gassan Shrines. The Osaka revival is anchored in the Imperial collection, where Sadaichi signed Teishitsu Gigeiin survive, alongside a Sadakazu made on commission for Prince . Across six centuries the track a single mountain workshop, from the subdued of the Dewa founders through a offshoot that turned toward , to the Imperial-rank carvers of Osaka who reproduced the old wave-grain while mastering the wider traditions.