This humanoid Gunmen was born when Gurren and Lagann combined using spiral power.
Most of its body is Gurren's, but adjustments were made to its limbs, such as extending them when it combined. Its helmet ornament was also stolen from Enki. The ornament was originally circular, but when Kamina stole it, the upper half broke off, leaving it crescent-shaped.
The fusion process began as a joke during a battle with Viral, when Kamina, enraged by Enki's "two faces, how rude!", transformed Lagann's lower body into a drill and forcibly attached it to the top of Gurren's head, calling it a "fusion" and resulting in a truly new robot (naturally, Gurren does not possess the mechanisms to combine with Lagann). This is due to Lagann's ability to control and grasp Gunmen, allowing it to take over Gurren via the drill, reconstruct its limbs using spiral power to match Gurren's torso, and absorb Enki's helmet in the same way. Lagann's drill, which had penetrated Gurren's cockpit, is exposed from the ceiling, leaving the combination as a bit of a forced one.
It is Lagann, together with Simon, the source of spiral power, that is important, and anyone can pilot Gurren (even unmanned) to operate it. Also, while the pilot of Lagann (Simon) generally serves as the main pilot, it is also possible for the pilot of Gurren to take over as the main pilot by delegating control (until episode 8, due to Simon's own inexperience, Kamina was the main pilot in most scenes). Furthermore, Lagann's takeover mechanism works without issue even in Gurren Lagann state, and Arc-Gurren Lagann was born from this mechanism.