Mt. Gassan is one of Dewa-sanzan[出羽三山（でわさんざん）](the three mountains of Dewa). Dewa-sanzan consist of Mt. Gassan, Mt. Haguro[羽黒山（はぐろさん）] & Mt. Yudono[湯殿山（ゆどのさん）]. Dewa was an old province name and that is Yamagata & Akita Prefectures now. These mountains are objects of worship of Shugen-dō[修験道（しゅげんどう）](a religion mixed Shinto & Buddhism). There is a shrine called "Gassan Shrine[月山神社（がっさんじんじゃ）]" on the summit of Mt. Gassan. Three mountains have each shrine. But it is impossible to visit the shrines of Mt. Gassan & Mt. Yudono in winter because of snow. For that reason, three mountains shrines are gathered & integrated to one shrine in Mt. Haguro and the shrine is called "Dewa-sanzan Shrine[出羽三山神社（でわさんざんじんじゃ）]".
Mt. Gassan is not so high mountain. But that is famous for Alpine plants and about 350 kinds of flowers bloom. Especially, that is a rare spot of Kuro-yuri[クロユリ] flowers(Kuro-yuri means back lily. That is called chocolate lily in English because that has dark brown flowers).
There is a large marsh called "Midagahara[弥陀ヶ原（みだがはら）]" at about 1,400m and you can see Alpine plants such as Mizubashō[水芭蕉（みずばしょう）](Asian skunk cabbage) and Nikkō-kisuge[ニッコウキスゲ]. And animals such as Okojyo[オコジョ](ermine), Nihon-kamoshika[ニホンカモシカ](Japanese serow) & Iwa-hibari[イワヒバリ](Alpine accentor) live in Mt. Gassan.
There is a ski ground that opens from spring to summer in Mt Gassan. The area around Mt. Gassan is one of the heaviest snowfall in Japan and much snow remains until summer in the mountain and a part of them doesn't melt all year round. And you can enjoy viewing red & yellow colored leaves cover the mountain.