And for that matter, how the hell is it an Eva ripoff? Can it even be considered a ripoff, since it was made by the same people?
Same production studio is not the same people, necessarily. I don't think a single major member of the Evangelion production team worked on Gurren-Lagann. That was mostly the team that did Abenobashi and Top wo Nerae 2.
That said, describing Gurren-Lagann as an Evangelion knockoff is basically insane. It doesn't use any characters, concepts, motifs, plotlines, or even visual sequences that are remotely like Evangelion's.
Gurren-Lagann is not a ripoff of anything made after about 1979 or so. What G-L was about was updating the feel of the peak Super Robot shows of the 70s for a new audience, with more complex plots and more characters.
Regardless of that, RahXephon's good...though you'll probably think it's an Eva ripoff as well.
I have to side against you here, good sir. Rahxephon reuses numerous visual sequences, plots, motifs, and character concepts from Evangelion. Japanese fans even broadly consider it an Evangelion clone and, generally, Japanese fans are more indulgent of shows ripping off other shows.
It's very weird because when BONES did Rahxephon they explicitly considered it a remake of Brave Reideen. There are some concepts and characters in Rahxephon that are 1-to-1 correspondences with Reideen. The emphasis on singing is clearly a stylish update of the concepts behind Reideen's famous God Voice ability.
I can only presume that at the time BONES produced Rahxephon, someone on staff decided that the only way to update a property was to make it more like Evangelion. It's a shame, though, all of the parts of Rahxephon that went into Evangelion's territory are easily the worst. If it had stuck to its guns as a Reideen remake I think it could've been a much better show.