The book charts the story of an elven city sacked by the Dark Lord, Morgoth.
The author started writing it in 1917, before returning to Middle-earth for The Hobbit and The Lord of the Rings.
Tolkien Society chair Shaun Gunner said many fans regarded The Fall of Gondolin as "the Holy Grail of Tolkien texts".
The Fall of Gondolin is the second "new" work to be edited and released by Tolkien's son Christopher Tolkien in two years.
It has come as a surprise to many, since Christopher - who is 93 - described the 2017 release of Beren and Luthien as "(presumptively) my last book in the long series of editions of my father's writings".
The only Middle-earth-based novels to be published by JRR Tolkien in his lifetime were The Hobbit and The Lord of the Rings trilogy. He died in 1973.