It's not copying because the stories are, in execution, characterisation, and most other details, completely different.
It's also just one comarison to one piece of epic literature. The point is that the classical epic is a mode of story telling (focused on grand themes, characters with impossible motivations, victory against overwhelming foes by force of will and perseverance as opposed to by superior ability, mortal men battling a supernatural world, and the struggle between men and gods) which Gurren Lagann emulates. It's not the story being emulated, but the voice, or the mode of telling.
I'll explain by way of example:
Star Wars is a fairytale. Yes, it looks like science fiction, but the actual structure of the story is a fairytale, which it tells you right at the start by saying "A long time ago, in a galaxy far far away..."
It features all the classic fairytale characters; the wise sage who lives in solitude, the abandoned prince unaware of his destiny, the imprisoned princess, the dark wizard, and the roguish brigand.
That doesn't mean Star Wars isn't original, in fact it's what made it original. Rather than trying emulate existing science fiction it took the trappings of science fiction and interwove them with the themes of fairytale fantasy (not, note, Tolkienien fantasy), to create something genuinely new and exciting.
In the same way Gurren Lagann takes the themes and structures of the classical epic, and dresses it up in the trappings of giant mecha anime, which in the process it brilliantly sends up, to create a story which is genuinely new and exciting, despite appearing to exist within an already overcrowded genre.