Mazinkaiser: Shitou! Ankoku Dai Shougun

Both anime are stylistically similar and have similar animation. They both have over the top action and hot-blooded main characters, and are related to 70's mecha anime (with Mazinkaiser being a spin-off of Mazinger Z, and Gurren Lagann having many references to old-school anime, especially Getter Robo and Mazinger Z). Overall, they both create a similar atmosphere and give a similar vibe. 

Since Ankoku Daishogun appears in Robot Girls Z, this movie that takes place after the events of Mazinkaiser should help you understand this character, and his army of Mycenae soldiers. 

Both are movies where the heroes are in Paris when a villain sealed for hundreds of years arises to take revenge on the world. 

Darker and Edgier sequels to Super Robot series that manage to one-up them in awesomeness. 

Yet ANOTHER remake of a Go Nagai favorite. The General of the Underworld is bad. Mazinkaiser needs to beat him up. It's awesome.