So... is Rokushin Gattai GodMars finished or not?
NO, it is not finished.
Why?
The anime finished after the manga:
Anime finished: December 24, 1982
Manga finished: January 01, 1977
... But MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
In the near future, humanity's expansion into space brings them into conflict with the Gishin Empire, led by Emperor Zule, who seek to conquer the galaxy.
Zule sends a baby named Mars, along with a powerful robot called Gaia, to Earth, intending for Mars to grow up and activate Gaia's planet-destroying bomb.
However, Mars is adopted and raised as Takeru on Earth, developing a love for humanity and resisting Zule's plan.
Takeru, possessing psychic abilities, joins Earth's defense forces, the Crasher Squad, to protect the planet from the Gishin.
Unbeknownst to the Gishin, five other robots were created to protect Takeru, and when Earth is threatened, Takeru can combine them with Gaia to form the powerful robot Godmars and defend Earth.
The story also explores Takeru's relationship with his brother Maag, who ends up on the opposing side of the war.