It is a bit ambiguous in this case.
We found unfinished non-core content:
Grendizer U: The Inception (Alternative Version)
Doronjo-sama wa Tensei shitemo Akuyaku Reijou no Mama Datta (Spin-Off)
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.
The mangas and the non-core contents are excluded by default, but you can enable them by clicking the buttons below:
Anime - Prequel
Release date: July 30, 2013
Anime - Base
Release date: January 04, 2014
Anime - Sequel
Release date: May 20, 2015
Robot Girls Z is a comedic action anime about a team of girls representing famous robots from the Mazinger Z series: Mazinger Z, Great Mazinger, and UFO Robot Grendizer. They call themselves Team Z and work to promote Photon Power, a clean energy source, in their local Tokyo neighborhood.
However, a rival team of robot girls, based on the Mechanical Beasts from the original series and hailing from the Subterranean Empire, frequently appear to cause trouble. Their goal is to steal Photon Power, leading to battles and comical clashes between Team Z and their adversaries.
While their main purpose is to protect the energy source, Team Z also engages in lighthearted brawls with the Mechanical Beast girls, sometimes simply as a way to let off steam, adding to the show's comedic and over-the-top nature.