Change!! Getter Robo: Sekai Saigo no Hi

Both are epic, MANLY and Hotblooded series about EPIC super robots fueled by mysterious green energy. 

Very similar aesthetics and themes. Armageddon is a very big inspiration to Gurren Lagann, especially in the final battle of the series. A big title in the mecha genre. 

I would more accurately recommend the various manga of Getter Robo, which should be read first to fully understand the Getter OVAs, but this is similar enough I suppose. Getter Robo is the most obvious of Gurren Lagann's many influences. The visual style of the battles in TTGL is reminiscent of Change!! Getter Robo, as well as the hot-blooded nature of the protagonists, and the similar concepts of Getter Rays and Spiral Power. Perhaps most blatantly, TTGL ripped the nature and motivations of its final villain directly from the later entries in the Getter saga, particularly Shin Getter Robo (the 1998 manga). The Getter also uses drills occasionally, if you're into that kind of thing. read more 

Gurren Lagann shares the same levels of plot insanity while omitting the "horror" and "psychological" aspects. While GL has a more comprehensive story, the action of Getter Robo Armageddon will satiate all the shonen bros out there. 

Getter Robo is Gurren Lagann's edgier and darker older brother. Both are ever escalating battles where ever more powerful mechas beat the shit out of ever more powerful monstrosities. And both mecha types have drills! TTGL has more story and mainstream cast, while GR is pure manly GAR testosterone. GR has less runtime and focuses on the fights. Which doesn't hurt enjoyment at all, in both shows. 

GORE, MANLY, WOMANLY, SUPER ROBOTS ULTRA RULE OF COOL 

Both are violent OVAs based on mecha anime from the 70s that re-imagine the original in a way that's completely divorced from their older versions' plots. Mazinkaiser SKL is honestly so much closer in tone to Getter Robo than it is to Mazinger Z that it's basically an unofficial Getter show. 

Mazinkaiser is Nagai's creation. Getter Robo is Isihikawa's creation. Ishikawa is Nagai's pupil and those two were the brains behind Dynamic Pro's success in the 70s. Both are excellent well animated ovas. 

Both of these shows revolve around three jets that can combine into three different super robots, but have drastically different plots 

Two Words: Real. Men. And plus if you like Wodan or Sanger you're gonna love this.