I think Gorden got it slightly wrong in the song ( It should be Gami, not Gumee)

According to the wiki,

The Ojibwe name for the lake is gichi-gami (in syllabics: ᑭᒋᑲᒥ, pronounced gitchi-gami or kitchi-gami in different dialects),[14] meaning "great sea". Henry Wadsworth Longfellow wrote this name as "Gitche Gumee" in the poem The Song of Hiawatha, as did Gordon Lightfoot in his song "The Wreck of the Edmund Fitzgerald"

But since Gorden's song was so popular a lot of folk assumed Gumee was correct.

https://en.wikipedia.org/wiki/Lake_Superior