So... is Gift: Eternal Rainbow finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: December 22, 2006
Manga finished: February 10, 2007
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
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Synopsis
Gift: Eternal Rainbow centers around Amaumi Haruhiko, a high schooler living in a town known for a constant rainbow overhead.
This rainbow is connected to a phenomenon called "Gift," a unique wish granted between two individuals.
Haruhiko's life is complicated by his childhood friend, Kirino, and the return of his non-blood-related sister, Riko, who had previously left town.
Riko's reappearance reignites their bond, and she, along with Kirino, enrolls at Haruhiko's school.
The anime explores the relationships between Haruhiko, Kirino, and Riko, all while slowly unraveling the mysteries surrounding the rainbow and the true nature of "Gift."