Here's something seen at Towards our Memories, written by a guest-blogger "SDS". Since ToM is not an ad-supported blog, I'm going to quote the gist of it:

There’s a hypothesis going around that each arc of the recently-finished Gurren-Lagann pertains to an era or decade of giant robot animation. I subscribe to this belief in a rather loose manner, seeing as how actual time tends to bleed and blur the borders and that people don’t all of a sudden decide to start thinking differently when a 9 becomes a 0, but the basic layout is this.

Episodes 1-8: The 70s, the original super robot era. (Mazinger Z, Getter Robo)
Episodes 9-15: The 80s, when things got more serious, when characters became deeper. (Gundam, Macross)
Episodes 17-22: The 90s, when perception was changed, things brought into question. (Evangelion)

If this is the case, then the current decade is the basis behind episodes 23-27. However, with nothing for Gainax to “reference” they can only instead try to give their own message, teach their own lesson, show their own influence.

Actually, there's more at the source, feel free to check it out. I'll just add that I had no idea that people took their thinking so far. It's weird, yet not completely hallucinatory.