If you're looking for anime similar to Tengen Toppa Gurren Lagann: Rasen Shounentan, you might like these titles.
The story follows an awe-imposing giant named Garon who comes from space to Earth, threatening humanity with extinction. However, a girl believes that Garon is not evil, and reaches out to Pick, a boy contained inside Garon's body. Meanwhile, the military plans to a nuclear strike against Garon.
The hero is Daisaku Kusama who encounters an enormous robot "Giant Robo 1 (GR1)" at ruins in Okinawa. He makes a contract with an ancient to be the commander of GR1. Daisaku is genetically modified to become one with GR1 to fight. The ally organization is "UNISOM." The enemy organization is "Gigantic Rebellion Operators (GRO)." They manipulate Giant Robos and start "armored terrorism" through the world. The mysterious girl in this anime is V. V is a girl who appears in front of Daisaku like an illusion. Her relationship between the GR is unkown.
Japan, ten years after World War II. Boy detective Shoutarou solves mysteries and fights crime with the help of Tetsujin 28, the 50-foot tall giant robot left for him by his late genius father. But one day he's shocked to learn that his father also adopted and raised another boy named Shoutarou, now a repatriated soldier. Just as surprising to our young hero is the fact that his “big brother” can also control Tetsujin, even better than he can. But with the emergence of this elder brother comes danger, as a mysterious assassin, Morning Moon, closes in on the boy for being unworthy of controlling the robot. Meanwhile, the capital faces destruction when multiple bombs are discovered, planted by the two Shoutarous' father himself!
AD 2019: the giant mecha known as LINEBARREL falls to Earth, grabbing the attention of the anti-machina organization JUDA. Before JUDA can arrive on the scene, an unfortunate accident occurs: LINEBARREL crashes into Kouichi Hayase, a weak and frail pushover of a boy. Waking up underneath a beautiful naked woman, Kouichi discovers that he now possesses great power and can summon LINEBARREL for his own purposes. All Kouichi has wanted is to become a hero of justice and hold his own, instead his childhood friends Yajima and Risako fighting his battles for him – and now he finally has that chance. With LINEBARREL's power, Kouichi becomes cocky and aggressive, lashing out at those he cares about most; and as a 'hero of justice', Kouichi must find his new place in the world and protect himself from two organizations who want their hands on LINEBARREL.
Once upon a time, a mysterious enemy, Unknown, suddenly appeared. The "Super Evolutionary Railway Development Organization," commonly known as "ERDA," develops a countermeasure: the Shinkalion, a robot that transforms the Shinkansen, in preparation for the threat. Taisei Ōnari, a second-year junior high school student, goes to Shinkai Gakuen Junior High School in search of clues about his older sister, who disappeared two years ago.