So... is Giant Robo the Animation: Chikyuu ga Seishi Suru Hi finished or not?
NO, it is not finished.
Why?
The anime finished after the manga:
Anime finished: January 25, 1998
Manga finished: March 01, 1968
... But MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
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Synopsis
In a technologically advanced future powered by the Shizuma Drive, a new era of peace is threatened by the machinations of the evil organization, Big Fire.
The International Police Organisation (IPO), a group of skilled fighters and superpowered individuals, stands as humanity's last hope.
At the heart of this struggle is Daisaku Kusama, a young boy who commands the powerful Giant Robo.
When the IPO obtains a unique Shizuma Drive crucial to Big Fire's plans, a fierce battle erupts between the two factions.
This conflict will push Daisaku to his limits and reveal a dark secret behind the Shizuma Drive, potentially leading to catastrophic consequences for the entire world.