Tengen Toppa Gurren Lagann Movie 1: Gurren-hen

While their length is different, both are super robot anime that take most of their ideas from classic super robot anime and present them in a funny and silly way. Both are self-conscious anime that are not meant to be taken seriously and give off a very similar vibe. 

While the plots of these two anime are different, they both have the same atmosphere and give off the same feel to the viewer. Both feature flashy colors, over the top action, GAR characters and manly tears. 

Both anime use common tropes that are present in other works of the same genre as their foundation, in order to expose and make fun of them (OPM uses tropes from the superhero genre, while TTGL uses tropes from the super robot genre). Both shows have a similar type of humor and over the top action scenes. They are self-conscious shows that try to be entertaining and are not meant to be taken seriously. They also create a similar atmosphere and vibe. 

Both are the weakest in terms of adaptation due to a distilled recap but serves as a gateway to the next movies. 

If you enjoyed Tenga Toppa Gurren Lagann (either the movies or the series), you will probably appreciate FLCL. Both use an absurd but well-done art style, have chaotic plot elements, utilize mechs heavily, and are strange/avant-garde. The overall message of each series is different (FLCL's being more about maturation while Gurren Lagann more about overcoming), but both are bildungsromans at their core. All-in-all, pretty similar. 

Both anime focus on main protagonist that live in a city that is detached from the real world. Also, in both anime, Earth has been turned into a wasteland where people have to struggle for their survival. The anime have similar dynamics between characters. They also have a similar mood, some fanservice and mecha fights. The main difference between them is that, at times, Gurren Lagann is a bit more goofy, and has a different art and animation style. 

Both are fast-paced anime, filled with over the top action and adrenaline-fueled moments. Both have similar animation styles and vibrant colors. Some of the characters are also similar, especially JP and Kamina, who have similar manners and a noticeable bravado. Also, both anime have great soundtracks. Overall, both anime have similar vibe, animation, characters and themes. 

Both are mecha anime featuring hot-blooded main characters and over the top action. They do not focus a lot of the plot, but choose to focus mostly on fights instead. Overall, they have a very similar vibe. 

Both are fast-paced anime, with similar art and animation, which focuses on quick movements and vibrant colors. Both have over the top action and unique mecha designs, as well as similar characters. They also have a similar vibe and atmosphere. 

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. 

Both anime are produced by Gainax and retell the events of their respective TV series. Both are mecha anime which pay homage to old-school series and contain many references. Story-wise, both focus on main characters who are thrown in a battle against powerful enemies for the sake of humanity. The main characters, Shinji and Simon, also have similar personalities. 

Both are part 1 movie renditions of great mecha series. I would say that both are a bit difficult to get into since some prior knowledge would definitely help, but hey, at least we get them down to movie form.